Oddly enough, that's one of the things I enjoy most about the Sinnoh games.
It finally made the Dex legit relevant without making it annoying by forcing catches like RBY. It also made trainer battles more relevant because if you battle every trainer in the game, you automatically get a full dex.
Ever since I had to run all over the place to slather honey around and get a Burmy to evolve into Wormadam only to bump into a trainer I had skipped near Lake Valor with one, I NEVER skipped a trainer battle.
Yeah I've never been a player who deliberately skipped trainer battles so I've always really liked that feature in Plat especially. It would be nice if you could get more specific hints to find the last few entries if you missed them, but overall I'm totally fine with gating postgame behind a fully 'Seen' Pokedex in a game where you can see everything via trainer battles (besides Rotom, which is absolutely an annoyance in Plat for new players). [EDIT: Suspicious Derivative pointed out that you have to battle Azelf and Uxie too, which I agree is also bad and annoying!]
What I really don't like is getting through an entire Pokemon game, looking at my Dex, and seeing that entire chunks are still blank. It's especially annoying in games where certain Pokemon are super overrepresented in trainer rosters; why not spice things up with some of the unseen Pokemon? It also sucks when a version exclusive gets this treatment, like how the Dex entries for Vullaby and Mandibuzz will always stay blank for someone playing Pokemon White, unless you trade for one.
Your mileage will vary of course: some people like the mystery of a fully unknown entry, but I prefer having enough information to find the Pokemon for myself, without having to guess at the relevant location or blindly evolve stuff.