If I already have Gamepass why would I buy the full game?
Isn't that the point of the whole service, that you don't have to? Or have I fallen for all the marketing spin?
It's a good thing I didn't pay anywhere near full price for all those months of GP.
I would argue the point of the Game Pass is to let you try any game you want as long as you're subscribed, the implication being that it's an amazing deal if you plan to play more than one game or discover as much of the library on GP as possible.
If you only really want one or two games and you plan to spend most of your time playing that, buying them outright is the better deal.
To illustrate what I mean, I can mention how I purchased Forza Horizon 5 as soon as it was available and I started playing since day 1 for premium players - November 5 2021. I have about 1,000 hours on it and I spent about 100 EUR since I got the ultimate edition. November 2021 was 19 months ago. If I had gotten the game with XGP, I would have had to pay for it every month, naturally. 19 months of XGP at the standard subscription rate is 190 EUR. Considering I can only really play one game at a time and most of my gaming time in a given week is spent on FH5, that would have been a terrible deal for me.
Now, if I paid the same price to discover a dozen different games over the same 19-month period, supposing they're all full-priced 60 EUR games, then instead of spending 720 EUR to buy each of those 12 games individually and play each one a little bit, XGP would've let me experience the same games for the same 190 EUR price.
TL;DR: In my sincere opinion, you get the most value for your money with XGP if you play multiple games. If you only really plan to use your Game Pass for one game, you're losing out.
Extra opinion: That very fact is also why XGP is absolutely not for me. With a subscription enabled I would feel pressured to get the most out of it, because every day I do not spend playing is a day I am not using my subscription for what it's worth.
I much prefer one-time purchases even if the barrier of entry is higher, because at least I can stick the game in my backlog and get to it when I get to it. Time not spent playing games on XGP is still billed all the same each month. Of course, that's just my opinion, it's what works best for me, your circumstances/lifestyle/etc may be different.
(edits to correct my maths)