Why bother with Degen at all, then? You need stats early so you can actually cast your spells, since Omni's base stats and stat growth are fucking terrible. You only want a single point in Repel because you don't want 16 seconds of not being able to heal someone (and being only 1 point keeps it nice and cheap to cast.)
By time you can afford to take Degen aura (usually around lvl 10 or 12), the laning phase is well over and chasing as Omni usually just gets you killed.
Again: Why would you take Omni over other options? Even if you wanted a heal, Dazzle does that better, he's ranged, Shallow Grave shits on Repel, and his ult can be damned near as good for pushing down towers (it's not invulnerability but giving your entire team +10 armor makes a significant impact.)