I have nothing wrong against them in spirit. The problem is that they are just
so
fucking
bad.
I'm a guy who can read some Umberto Eco and then chill back with some (say) Jim Butcher, but every novel I've ever read or skimmed that could be considered a tie-in was written in a style that was about as dry as an air-conditioned crackers factory in the desert. A computer hooked up to a fantasy database could do a better job than them. I know that it's because writers who write such things have to adhere to ridiculously strict guidelines, and so I don't blame writers, but the entire set-up including writers. If you force your writers to follow guidelines worth of a reference manual don't be surprised if the resulting work is as amusing as a reference manual.
To be honest that is common with fantasy fiction as a whole, but tie-in novels bear the brunt of it.
Speaking of which, the other day I saw a Metro 2033 novel on the library. It said the game was based on it and not the other way around, anyone know if it's any good? I think I haven't read a good post-apoc novel in like ever.