I say this a lot, but there seems to have been no reason to have select two gym leaders end up as main characters, except that maybe the writers were lazy coming up with designs for party members. Besides, when you think about it, Takeshi joining up with Satoshi in the first place didn't make much sense (I'll give Kasumi the benefit of a doubt because she had her bike trashed, and in the anime continuity, she wasn't exactly a gym leader anyway, not until later on).
Munou: "Hey, random trainer that came into town, I mock you if you think you can take on Takeshi, even though he's a gym leader and loses to millions of trainers like you."
Satoshi: "I'll show you.... OH! NOES! I GOT PWNZORED!"
Munou: "Man, you took quite a beating.... I see potential in you. I'll help charge up your Pikachu."
Satoshi: "Yay! Now my Pikachu is all uber and stuff.... but not really.... but I set off the sprinklers that this seemingly incombustable building just happened to have anyway, so it doesn't matter. Now I'm gonna pwn Iwark.... but not really, because those sprinklers were cheap after all."
Takeshi: "I got my a** kicked by millions of trainers before you, and yet you just happen to be the one person in existance who showed me true sportsmanship. Here's your badge."
Satoshi: "Yay!"
Munou: "I am really his father, and I'm going to return to my responsibilities with the family. I didn't want to before, because I was just waiting for my son to admit defeat to you, random trainer #1907268 whom I just so happened to see more potential in."
Takeshi: "Good, now get to work you lazy s***! Wow, now I'm free.... I'm going to join up and travel with you, random trainer whom my father saw potential in. I'm going to pursue a career in Pokemon breeding, because it was the best excuse the writers could come up with for me, a former gym leader, to bother travelling with the likes of you, a rookie. And in the long run, if you're lucky, you won't even have to deal with my pursuit of this goal and I'll be little more than your personal cook and cheerleader."
Satoshi: "Yay! I got a cheerleader. In yo' face, Shigeru!"
Yeah, that's pretty much it in a nutshell.