How about "it depends" as an answer.
Generally, the more people you train with the merrier. However, if you want to work on something specific or feel something specific then by all means try and grab a partner that can help you feel or work on what you are working on. Some will be better than others for certain things. For instance, sometimes I want someone that will fight and resist me every step of the way and sometimes I don't want any resistance at all. Or, if you have no clue as to how to do a technique it's possibly more prudent to grab someone who does than a first day on the mat white belt.
The problem with working with the same people all the time is that your habits and patterns get ingrained, comfortable and never threatened. It's sort of like germs and dirt. It seems, from what I've heard, that putting children in a safe and clean enviroment weakens their immune system because it's never tested or exposed. Whereas letting them play outside and get dirty strengthens their immune system precisely because it is tested and exposed. You don't want children in a completely safe and sterile environment. Seems to me that Aikido is the same way.
Most of the time I don't care who I work with but not always.