Well, regarding the OP, fun is arbitrary. Some people can't enjoy losing at all, regardless of why or how, and if they're not winning, they aren't having fun. For those people, picking a top tier is what brings them the most fun. I had a friend like this when I played Smash competitively who dropped all character loyalty he had towards ROB when introduced to the competitive community, and the tactics that made MK the best character.
This same player claimed to love all fighting games at a competitive level, but when the rest of the local scene moved on to other games, he chose top tiers, got bodied (Because they weren't AS good as MK was in Brawl), and promptly quit the games since he wasn't having fun as he wasn't the best.
Conversely, you have players who enjoy the challenge of playing with low tiers, and have the most fun playing bad characters but outplaying their opponents and winning with said characters.
You also have players, of which it sounds like you may be one, who pick their favorite characters and stick with them due to character loyalty. Beating their opponents with their favorite character makes the game fun for them.
Finally, you have those who like certain playstyles, and stick with those playstyles regardless of who the character is or what their tier placement is.
Of course, then you can mix and match all of that, but what I'm getting at is what may seem like someone betraying their principles to win may just be their way of optimizing the fun they have with the game.
Personally, I tend to go mostly for playstyle, but also try to have a high/top tier counterpick character in most games because the characters who's playstyles I tend to like usually tend to be low tier as all hell (Mario in Brawl, Ezio in SCV, Captain America and Chris in vanilla Marvel, etc.), although they also tend to be the ones I have the most fun playing, especially if no one else is playing them.
