I think the nature of the campaign dictates how retrain rules work. In LG, your primary character was a big deal because it was you face in the community over 5-8 years (people knew each other's characters and the PCs' deeds, mannerisms, etc.). Retrains of something like class and race would have hurt play in that campaign.
In LFR, retrains really weren't a factor beyond things like people abusing new rules or shifting ability scores. That's really pretty minor.
At the same time, most times I try out a new game as DM I like to let players remake their PCs over the first few session. A number of organized play campaigns do this. It lets you join in without fear that you are locking in your choices. Make a PC, play, and if you really aren't happy you can make a change. Most players won't do it. Most players into the story also won't make the change.
If the first levels go by quickly, or if the seasons of play cause PCs to be set aside after a year or less, then that retrain period won't make a big difference.