To ingolf: Well, each to his own, I guess. But if you want to mix music that translates well over various listening systems, it makes some sense to have speakers and a room that you can trust.
Yeah, you can always fix it in the mastering
But only to a certain degree.
The better you can judge what's going on in a mix the more creatively you can mix or master instead of having to iron out errors.
I sometimes mix projects from friends, recorded an composed on not soo good speakers in untreated rooms, and instead of polishing the sound from already good to great I often have to try to make it at least work somehow.
When they are over in my (not professional) room, they then often say: ohhh, so that sounds different than I wanted it to be
Just my 2cts