Wasnt Chuck Lorre the original producer of Cybill Shepherds sit-com Cybill, who she very publicly sacked, so im guessing it was his way of getting his revenge.
As for Will and Grace, though i agree Karen and Jack stole the show, it would have been too much just to have them, they needed two less extreme characters to really make them funny.
Someone mentioned about Friends really trying to push Monica, but later changed to the Rachel show. Courtney Cox was seen as the star of the show when it was first pitched to NBC. However, they originally wanted her to play the Rachel character, but she only agreed to be part of the show if she could play Monica. So it was probably a case that Rachel was always ment to be the main character, they just tried to change the focus in the first series more on Monica, as they saw Courtney as the star. I wonder if Courtney Cox ever regretted her decision (especially in the mid seasons when Jennifer Aniston was such a cultural hit).