What are the side effects of raising the PROCESSES system parameter?
At least it has you have to increase most of the OS kernel parameters(like number of file handles). Most of them have a formula where use PROCESSES is an input. Also each server process allocates some RAM - the about is not big, but when multiplied by one thousand ...

Idle processes do not have any major overhead, but when DB server starts swapping RAM it's over. All the performance is away.