Why do so many aikidoka of the modern day go play with judoka, BJJ, and MMA players?
I think there are three reasons:
1. Judo, BJJ and MMA are all sports. Sports have rules. Aikido violates directly or indirectly most of the rules that most of these sports have and so Aikidoka "loose" against them.
For example, there is no way to beat a Judoka unless you're a better Judoka or you violate Judo rules, so in a Judo dojo you will always loose.
Most people seem get stuck on the fact that they lost and don't give much thought as to why they lost. They don't blame the other guy because he won, they don't blame themselves because that's too hard, they don't blame Judo or whatever because again they're the one that lost, so they come to the conclusion that Aikido is inferior. The common sence thing to do then is study the superior art.
2. Other arts provide a testing ground for what you've learned. You can go up against a fully resisting opponent without having to start a real fight.
3. A lot of Aikido people tend to be the kind of people that habitually do more than one art simply because they enjoy learning other arts.