The fact that more or less _everyone_ that installs xemacs installs the sumo package too should tell us something about how useful separately distributes modules are, though.
The main advantage of pike is that pike is pike, given a specific pike version you can be fairly certain that the modules are included (such as bignum, image handling etc).
I can see that it might be advantageous from a maintenance perspective to split up pike in multiple modules. Doing so requires a module owner for each module, that actually maintains it, though.
Currently the core developers are the only ones developing pike actively. I don't see that changing just because each module is split into it's own CVS repository with separate releases.