Collecting doesn't necessarily have to have anything to do with the quality of the books.
Tie-ins were probably important in getting me into reading - I remember reading Doctor Who, Star Wars, Star Trek and Flash Gordon novels in my youth - but I kind of grew away from them. I became a bit more precious about canon, and kind of felt like I was wasting my time reading things that might not even be real in the context of their own fictional worlds.
Back when I used to do a lot of tabletop role-playing, we rarely used the pre-made worlds, and it seems to me that Forgotten Realms is designed to be generic and kind of uninteresting. Dragonlance has the opposite problem, that it was largely designed to tell one story, and doesn't work that well as a gaming space. The first two Dragonlance trilogies are the only tie-ins that I still feel might be important. I see from wikipedia that there are now over 190 Dragonlance novels, which is frankly ridiculous. It does seem incredibly exploitative when you get up to those kinds of numbers.
I don't know really. There can definitely be good books which are tie-ins, but it does kind of feel like commercialised fan-fiction. And is commercialised fan-fiction even a bad thing?