If Mythic level attributes aren't going to be allowed normally, I don't see why they'd be fair game for Modular Abilities. You're still buying them, just for temporary use.
In all fairness, you're technically right. But they're fair game anyway, just advised against.
Worst-case scenario though, I just don't ever use mythic-level versions. I'd just bounce between having one at -4 and one at -2...or all three at -2...or maybe even something else if it suits the character.
I'm not sure if you actually meant it this way, but you don't pick a list of abilities you have access to when you take Modular Abilities...just set a total point pool for the maximum allotment you want...and add two to the total for the cost. I'm merely choosing to artificially restrict myself to strength, speed, and recovery...instead of having access to all Creature Features, some Minor Abilities, and all of Strength, Speed, and Toughness. If that wasn't what you were implying, then ignore that whole bit.
Someone else mentioned transforming into a different animal or creature relevant to whichever power I'm using through my Modular Abilities being a requirement. That's not how it works. Modular Abilities means you only shapeshift the parts of yourself that need to do the job, and only so much as needed, and nothing else. Boosting my strength means my muscles might bulk up a bit...speed could mean they'd get much more toned...recovery probably means I just generally heal faster, perhaps increased metabolism at best. The only overt shapeshifting one gets from Modular Abilities is if you ever use claws or wings or the like. Modular Abilities itself doesn't allow you access to the other shapeshifting powers at all, and it has no Musts regarding taking other shapeshifting powers first.
Perhaps it should have a must of some kind though. Not sure what, but ah well.