Yes, thank you Sherk for clarifying exactly what we'll want to avoid at all costs.

Guildwar's NPC teammates horrified me to the point where I still can't really consider the game an MMO.
To give a more concrete example of what I had in mind then: Imagine a 1 star NPC caster who can cast exactly one spell at a quarter the efficiency of a PC, and in complex situations gets in the way more than he helps. A fun little addition to play around with and to make soloing a little more bearable to those who insist on doing it, yes*. A viable alternative to a fully-controllable and versatile alt character, let alone another player? Not even close.
*Personally I think soloing is fine, I just want these things to play around with, ordering them around and getting them killed a lot.

But it's a common complaint I hear from people who prefer to solo or can't find teams at their playing times. The ones amongst this group who can will probably just get a healer-alt, but not everyone can.