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It's something I keep in mind when choosing the game, because it may affect my team composition. But that only matters when one or more of those exclusives end up being part of the team. I pay more attention to this in games with a smaller Dex, since the team building options are more limited.
A little, but not tons. Represents a small number of Pokemon in the pool. Typically finds a decent alternative elsewhere.
Prefers no version exclusives (and a single game, for that matter). Encourages trading with things like starters and fossils. Why not offer more choices like that in-game, rather than an out-of-game purchase?
A lot actually. Exclusives always determine me picking what version I want because the main differences between games that interest me come down to their exclusive pokemon. For instance, I really like the Misdreavus line and I thought Ceruledge was much cooler looking than Armarouge (as well as the future paradox mons looking cooler than the prehistoric paradox mons) which is why I bought Violet over Scarlet. Exclusives are also the main reasons why I would replay a previous gen pokemon game as well. Most noticeably in my recent playthrough of Sapphire as I went out of my way to include at least 2 exclusives such as getting a Ludiocolo (not obtainable in Ruby) as well as a Muk (25% spawn rate in Sapphire versus Ruby's 2%). I didn't get any specific Ruby exclusives but I did get a Medicham which is unobtainable in Emerald, and Emerald is the last of the gen 3 games I will beat someday.
The only time exclusive pokemon were not my main focal point of interest was in gen 8 because depending on whether the title had "sun" or "moon" meant the game's timecycle would be normal or inversed. I eventually got Sun and Ultra moon to experience that difference.