Laura was really one of the best characters on the show, but she was sadly underused. I agree she was underused after Season 4, at least for the most part.
There were times she slid in and was used more significantly, especially when working with Abby or when she started seeing Greg. But there's a stretch there between Seasons 6 and 8 where she's only used in every other episode.
It's sad, too, because Constance McCashin is a really good actress, but it's obvious the writers just weren't terribly interested in writing for Laura after a while. The concept of "four couples on a Southern California cul-de-sac" faded relatively fast -- Kenny and Ginger and Richard were edged out, and while Laura hung around, she wasn't used nearly enough.
Maybe they could've had Laura fake her death and brought back McCashin in the last seasons, but she probably would've declined, and the budget wasn't there anyway. I doubt I would've liked that anyway.
