Eviolite is practically mandatory in ZekEels. :\
Not even close. I use to be a huge proponent for Eviolite in Eels, but I don't feel it is necessary anymore. Nice to have? Yes, most certainly. But mandatory? No.
Let's think about situations when Eviolite is useful. Against Terrakion, it obviously does have use. Reducing 20 damage from your Mewtwo EXs or Tornadus (if you play them) can be huge against a Terrakion deck. Really though, that's the only way it is useful in that particular matchup.
Now we'll move to the mirror. It is, honestly, not that useful here. Eviolite on a Thundurus is pretty pointless. It does require a Zekrom to have a PlusPower to OHKO it, but that's really all it does. It's nice reducing your selfdamage on Bolt Strike to 20, but again, all it does is require an opposing Zekrom to have a PlusPower to OHKO you. It's mostly useless on Mewtwo EX in this matchup; the only way it helps is against a Thundurus attacking a Mewtwo EX, but if you have to attack a Mewtwo with Thundurus you are already in a sub-par position. Eviolite is just a 'win more' card in that situation.
NOW, I will give you that it can be useful on a Zekrom EX in this matchup, but as I said before, I don't even like Zekrom EX and I do not run it at all.
Now we'll move to the last matchup that actually matters, that being Celebi. I don't find it that useful in this matchup either. Yes, it can require the Celebi player to have 1 more energy on Mewtwo EX to KO you, but that is basically it. It's useless on your Mewtwo's and only really matter on Thundurus and Zekrom. While it helps here, Celebi is most threatening in the early game, where Eviolite is irrelevant (mostly). When you take Celebi to the last game, you are already better positioned to win simply due to the way Eels functions, so Eviolite is, again, a 'win more' card.
So yes, Eviolite is handy and can be nice to have, but it is in no way required. Depending on your build (and it IS build dependent), you may not need it at all!