Okay, I don't see these benefits because my stadium is smaller...so I make it bigger and maybe fail to make the WCL since my country only gets one spot. Seems like a risky investment until the league champion is sealed, but it's something I'll be sure to take it into consideration when that (hopefully) happens again.
Edit: I also just noticed that player condition is decreased half as much from WCL matches than for friendlies. I guess that's another incentive: playing a player with 94% condition in the WCL is equivalent to playing a player with 98% condition in a friendly. That makes me feel better about using my better players for the WCL, even if they don't have 100% condition.