That the book "reads" like Heinlein makes my point: A writer with a distinct, individual style wouldn't be spending his time mimicking someone else.
I dunno...Todd McCaffrey seems to pull off Anne McCaffrey's _Pern_ very well, at least from the two stories I've seen so far.
Some of the best of MZB's _Darkover_ books were the ones released a few years after she died, co-written by Deborah Ross from MZB's notes and outlines. The style is like the best of MZB. I've even seen a fanfic written by a random nobody who likewise picked up her style perfectly.
Really, for me a co-author or fanfic author is judged on how closely they can pull off the original author's style...it really becomes a part of the way the world itself is presented, at least for the worlds I know best.
Roleplays are another extension of this. Bending canon a little is one thing...female blueriders in Pern, for example...but when roleplayers "rush" the story or completely trash any feel of it being canon Pern, that ruins it for me.