As was mentioned right away, the boss refights are a great opportunity to be able to play with RM weapons and figure out every weakness without having to look it up online or play through the game several times. I don't particularly prefer the X1 method over the teleporter rooms, in terms of pacing, but removing refights altogether would be detrimental for sure.
I am, however, a big anti-fan of the Doc Robots. If I wanted to fight the bosses from the last MM game, I'd be playing the last MM game. I don't want to have to play through recycled stages to fight recycled bosses because the designers were too lazy to come up with something new. Even in X1 or after every teleporter room, there's still a new boss to fight when it's all said and done.