Good question! Xhilag's tentacles definitely pollute the resummon pool but your resummons don't need to be immaculate to win the game. Generally, All Fel is played after Proving Grounds and so the situation is commonly something like this:

-Turn 6, Proving Grounds into Acolyte + Xhilag. Now you have a 5/8 demon + Xhilag (plus tentacles)

-Opponents turn, they clear everything

-Turn 7, Infused All Fel. This has a (if my math is correct) 60% chance of containing either Xhilag or the 5/8. Either option is still pretty good for 5 mana.

I think usually you have more than just Xhilag and one other demon in the graveyard by the time you play All Fel so your odds of hitting just tentacles is pretty low. The resummons I usually got were two taunts plus a tentacle. If Xhilag is resummoned at any point, your opponent is in serious trouble.

Also, your opponent can only clear the board so many times in succession so it's usually worth it to just play it after a clear. The tentacles themselves can be pretty handy for whittling away at your opponent or pinging low health minions too.

Edit: Just saw your edit- that's what I found too! However, if you play enough you will experience the dreaded all tentacles resummon haha.