No, PlusPower and Strength Charm definitely do not need to be in all decks. Of the two, PlusPower IS by far better, as it's not a Pokemon Tool, AND is cumulative, that is, you can have more than 1 working for you at a time. PlusPower isn't allowed in Modified, however.
In Unlimited, many decks used to use them, and a number still do. One that comes to mind is RainDance. Hydro Pump maxed out at 60 damage, and those extra few PlusPowers in your deck would take out those Clefables, Scythers, Hitmonchans, etc, that had over 60 HP. Also, they help with cards like Sneasel, that have to get a number of heads. If you would play 2 PlusPowers, that's one heads less Sneasel needs for the same KO. (This isn't the greatest thing to do, so you really only should do it when you HAVE to get the KO, or otherwise, if you could win really easily by doing so.)
In Modified, Strength Charm is here. As I mentioned, the most you can use is one of them, for a max of +10, which isn't that great, but when you slip a few into your deck, you can surprise your opponent. I remember an E-on game I was playing a little while back. My opponent used a Strength Charm on an Expedition Machop. I had an undamaged Electabuzz ex up. What do you know, he flips both heads for his attack. 40 damage times Weakness plus 10 was a KO. @_@ One attack from a little Machop took out a Buzz ex with 90 HP. Without the Strength Charm, he couldn't have KO'd me.
Sometimes 10 damage makes all the difference.