Would it matter? 99.9% of all damage output is from you. You just need the aura.
It does kind of matter. A properly geared merc can put out plenty of damage and can be handy. In my gameplay with Clervis's "Drop Bear" enchantress build (and if I remember correctly, my merc's gear is essentially identical to his merc's gear in the guide), I've deliberately used my merc to clear several dungeons (on /players8) just for fun. I buff him with Enchant and follow him around letting him kill everything, occasionally teleporting us to new positions to speed things up. That's a silly conceit on my part and I could clear those areas faster by transforming into a bear and hitting monsters myself, but it's pretty easy and the merc doesn't even get into trouble that often.
For almost all characters, the player should be contributing far more damage output than the merc, even if the merc is optimally geared for damage, but your figure of 99.9% is probably in a rather extreme part of the spectrum. Not impossible, but I'd say it's highly unusual. Most characters use mercs to deal
some damage, whether it's mowing down weak monsters that aren't worth it for the player to target directly, dealing with immunities, applying CB to bosses, blocking a pathway and holding off the monsters coming from that direction while player kills the monsters in the immediate vicinity, or just randomly picking off monsters that happen to be in the merc's path while the player is focused on something else.
Even if your playstyle is such that your merc really does generally feel like just a walking Conviction aura (perhaps a really optimized javazon that can pack enough physical damage into Jab to slaughter lightning-immunes without help), it is worth it for the merc to have a good base weapon for Infinity so that the merc can better survive through LL when he gets swarmed and you need that extra second to help him out.
I did mine in Eth Thresher since I think its the most versatile base.
Thresher is usually the right answer. It's the fastest, so it procs CB more often.