You enjoy competing in a video game, big deal. What's there to justify to anybody (let alone some icon connecting you to a bunch of anonymous strangers on the interwebs (i presume... the only console i own is a GC, so i don't really know about that sort of thing))?
Well, regarding rainbow ride... playing competitively and horsing around at times isn't mutually exclusive. Just because you play with the competitive ruleset doesn't mean you can't ever do anything else. If you think Bowser dittos on Poke Floats with bombs on very high sound fun, go ahead do those.
It's just that the majority of the playerbase thinks a largely static environment is best to determine the better player, so we do that in tournament sets... but, if you and your opponent agree to sth, next to no tournament host is going to prevent you from doing whatever the hell you want as long as you don't stall the tournament (set the timer higher or play more matches than your set should be, or sth).