I'm curious as to how you out performed your summoner though as an arcanist. Did you mean in overall usefulness or pure DPS? If it was DPS then that shouldn't be happening even if you could pick up BfB and IR.

First of all, you are gimping yourself prior to entering Coil due to the myth sets being job only, though this is lessened somewhat with CT loot.

Secondly, you'll be unable to wield your relic weapon.

Thirdly you lose your job abilities. Pretty big loss in DPS since you lose Fester, Spur and Enkindle. Furthermore, when equipping your soul of the summoner you get increases to all your stats so your potential heals increase if you're going for diversity and you keep your battle rez and all that.

All in all, base classes aren't useless, they're just generally less effective at endgame when considering what their jobs are specialized to do. Plus you can't queue as a healer as arcanist. 30s-early 40s I don't think anyone should really care about what other people are using.