Melanie Griffith is a regular on the WB show Twins. I didn't watch for her though. I was watching for Sara Gilbert (Roseanne) and Molly Stanton (Passions). I was shock to see Melanie Griffith in the credits though.
Acting is acting. Some are better in different venues, but ti all comes down to going where they are willing to hire you. Some choose Tv for the opportunity to take on a challenge and some because it's a better schedule for their families. I agree that if they are cast for what they can bring to the show, it can be really great, but the stunt casting of guests needs to be kept to a minimum. I remember during the first couple seasons of Will & Grace (when I still watched regularly), those appearances were only every once in a while, but eventually it seemed like the had a big guest for every single ep (sometimes more than one). That is a bit much. ER has always done a decent job with guest stars though.