Serious league's use bidding sytems.
I was all for this at one time and play in a league that had this system in place until the guy with the deepest pockets and most money would make outrageous bids to pick up all of the good players. Needless to say, he won the league but didn't earn too much when it was all said and done. It just made the league no fun because anyone with any value he would get because he was willing to spend an outrageous amount each week and had more money than everyone else.
We have since gone back to a WPS and the league is now very competitive. The guy with the deep pockets is 0-4 since he didn't draft well (as usual) and can't buy a good team.
I wouldn't say that only serious leagues use a bidding system as this is not true. The leagues I play in are very serious and none use the bidding system.