Something that you have to remember is that there may be a lot more to play than just releasing cards Japan has gotten. With the release of the next Generation happening at the same time worldwide, the Trading Card Game has a bit of an issue being multiple months behind in sets. This may cause the releases of English sets more rushed than we would like, and less time being spent on how the specific Pokémon lines released interact with the Pokémon lines of sets yet to come. TPCi was probably more concerned with where a rotation needs to happen in the future than whether or not the Taillow in this set can evolve or not.
As for having the incomplete evolutionary lines in the sets, this is a bit more of a trickier situation to fix. It would need a considerable amount of foresight in order to rectify early enough
Each Japanese deck, mini set, promotional, and gift set release is designed the same way. They are designed to span the course of about three or four sets when they are released. Let's look at the evolutionary lines that were incomplete in Boundaries Crossed, and where they come from
ikachu: Japan gets Pikachu all the time. It's a really hard Pokémon to complete the evolutionary line for when they are released in English. Japan currently has 8 Pikachu in the BW on format (and this does not include the Jumbo Pikachu). They also only have 2 Raichu in the BW on format. Having a 4-1 line of any Pokémon is extremely ridiculous. Sometimes a lone Pikachu needs to happen.
:Chinchou: Chinchou was designed to go forward 3 main expansions, with the Chinchou/Lanturn released in the Japanese Spiral Force. When Boundaries Crossed was released, we didn't know these two cards existed. You also have to allow time for set printing and distributing, and you have an extremely large time span and necessary foresight in order to get this 2-1 line together in a set
:Gothita: Gothita and Gothorita were designed to go with the
Everyone's Exciting Battle pack Gothitelle OR the Dragons Exalted Gothitelle. The English creators of sets would have either needed considerable time and foresight to prepare for the National Beginning Set's 1-1-0 line and translate them earlier for Dragons Exalted, or, they would have needed a lot more insight to put it with the Exciting Battle Pack line.
:Makuhita: Same exact situation as Chinchou, but the Makuhita/Hariyama pair of cards was released in Thunder Knuckle instead of Spiral Force. 3 sets worth of preparation would have been necessary, which would be difficult to ask since they probably aren't provided by Japan with that amount of information that far in advance.
:Klink: Designed to go with Plasma Storm's Klinklang. Only 3 months of foresight would have been necessary here, but TPCi was probably more worried about the fact that they need to release about 650 cards in 4 sets before a Generation 6 set needs to coincide with the release of the games in October 2013. Having a Pokémon that can't evolve is not the end of the world.
:Meowth: A similar situation to Pikachu but less extreme. There are 4 Meowth in format and only 1 Persian. Even 3-1 lines of a Pokémon are considered ridiculous. 4-1 is out of the question.
:Togepi: Needed to wait only 1 set, but it didn't. Mistakes happen. Lone Togepi doesn't end the world.
:Taillow: It's a lone Basic in the Japanese format. There is not a Swellow released in BW in any language. There probably will be a Taillow Swellow line in Japan's March set Megalo Cannon, but we wil just have to wait and see.
There are a few more honorable mentions from the national beginning set that I feel should just get a word.
:Croagunk: Correctly placed with the Freeze Bolt/Cold Flare Croagunk/Toxicroak
:Mienfoo: Correctly placed with the Freeze Bolt/Cold Flare Mienfoo/Mienshao
:Voltorb/Electrode: A full evolutionary line that could have worked anywhere, but probably should have been saved for the Electrode that has Magnetic Draw released in Thunder Knuckle. It would have provided a fuller evolutionary lines for drafts and prereleases and would have enhanced these events.
Keep in mind that they also needed Pokémon for a half lightning theme deck. 5 of this set's 8 Lightning Pokémon were available in the IceShock theme deck. When you remove too may of the Basics from any type, suddenly the Theme deck will not be as good, and that is not the direction we want those to go. The Theme Decks have been made considerably better as of late, and I don't want that to be ruined because someone thought it necessary to complain about something different without considering the side effects.
Additionally, 8 incomplete lines is not the worst it has ever been. One set that comes to mind is Neo Revelation. That set had a Skiploom/Jumpluff no Hoppip, Quagsire no Wooper, Kingdra no Horsea/Seadra, Raichu no Pikachu, Flaaffy/Ampharos no Mareep, Geodude/Graveler no Golem, Aerodactyl no Fossil, Houndoom no Houndour, Magneton no Magnemite, Snubbull no Granbull, Porygon2 no Porygon, Blissey no Chansey. This set had 12 incomplete lines. and this set was out of 64. 8 out of a 149 set is a much smaller ratio than 12 out of a 64 set. Additionally, at least every incomplete line from Boundaries Crossed could be played due to them all being basics and stage 1's. Jumpluff, Skiploom, Quagsire, Kingdra, Raichu, Ampharos, Flaaffy, Aerodactyl, Houndoom, Magneton, Porygon2, and Blissey were all evolutions that couldn't be played at all without cards from other sets.
Also, consider the only set during Black & White that has 100% complete evolutionary lines: Emerging Powers. They probably worked hard to bring together so many broken families for that set. The cards from EPO came from the Japanese Beginning Set, Black Collection, White Collection, Cobalion, Terrakion, Virizion Battle Strength Decks, Psycho Drive, Hail Blizzard, Dark Rush, and the BW-P Promotional cards. That is 10 different sources those cards came from. Most sets only get about 3-5 different origins for their cards. It probably took a lot of preparation, time, and effort to collaborate such a Jamboree of a set. Additionally, remember that Emerging Powers got an extreme amount of complaints because of the cards that were included in the set. Why should TPCi spend so much preparation and time to make a set 100% complete evolutionary lines when the last time they tried to do it, they got so much flack and complaints? It is soooo mind boggling and hypocritical to both complain about a set not being the best when they actually try to fill every evolutionary line, and complaining about broken evolutionary lines.
There's a lot more at play than just the English creators releasing a bunch of "bad cards" in sets. So much more needs to be considered before blindly complaining about such a trivial situation that the PTCG has.