I remember when I first saw The Muppets Take Manhattan. They had the wedding at the end and some of the cast of Sesame Street was there, and I was like, "Oh my gosh, it's the cast of my favorite show in this movie! This is the best thing EVER!" I was mega impressed.
Of course, I was five years old.
I find it really hard to believe anyone much over the age of five could be that impressed by these crossovers. I actually find them very irritating. Like when I'd be watching Empty Nest and all of a sudden, one of the Golden Girls would pop in for no apparent reason (other than to get a ratings boost).
In my opinion, the ONLY show that ever did it well was The Simpsons. They were self-conscious enough about the crossovers that it was actually funny.
The reason I'm whining about it now is that I've been noticing it more and more in books I've been reading. Emily Giffin did it in her last two novels, and it's beginning to really piss me off.
If you are writing a sequel to a book you wrote, then fine, you can use the same characters. Otherwise, it's obnoxious.
For example, if Rose from In Her Shoes is walking her dog in the park, I don't want her to run into Cannie from Good in Bed and have a little conversation, just for fun. This has nothing to do with the plot. It's as tacked on as Jennifer Hudson in the Sex and the City movie.
I don't even know why authors do that. It can't help sales the way it helps TV ratings. Frankly, I think it's a form of masturbation.