http://variety.com/2014/tv/awards/guest-stars-mad-men-blacklist-modern-family-1201212599/

“Early days in the series, we had a lot of names on, and there was kind of a backlash, like, oh, is this turning into ‘Will and Grace?’“ says casting director Jeff Greenberg. “The show didn’t need to rely on guest stars, and we really want to choose the right person for the part.”
Read more at http://www.hitfix.com/tv-tattle#VXbQX6fI0FBh8k8J.99

I don't think Will and Grace relied on famous guest stars. The writing was always really good and celebrities were always asking to be on because they were fans of the show. So they wrote them in.