Most of the time I wind up using only 2 or 3 unless I'm feeling sporting and what exp on my other guys. So I kind of doubt that would really make things harder since, well, you're likely to have that many pokemon that can handle them. Just about the only way I can genuinely see making the leaders harder in a way that's "fair" is to go with Battle facility rules. 3 Pokemon, Set switching, no items, fixed levels, etc. But that's riddled with a myriad other issues.