Very interesting question that I have wondered also. I personally think grinding such games with right mentality can be fun and unlocking new stuff and features is cool.
Few things I would consider however
Games like Pirates Plenty ( and some others) give you the feeling you are grinding in order to unlock new feature that gives you better chances to win big.
I'm not really sure if this is the case. I mean sure you unlock the wild reel in Pirates plenty for example. Theoretically this gives you the chance to win bigger wins. My question is however does unlocking the wild reel affect the rtp or are the wins balanced some other way? Game only lists flat rtp of 96,12%. I have seen many games with either rtp range, or two separate rtps listed. So why not here?
Wild swarm is also really interesting game. I was first a bit surprised when I learned that you can upgrade your bet size after you have filled up the hive. I was curious how are they going to balance this when you could theoretically fill up hive to lvl 5 with minimum bet and then trigger swarm mode with max bet right after. Notice game lists flat rtp of 97,03% > So I came to conclusion that the whole bee collecting hive thingy is actually just a sort of meaningless visual hoax. It doesn't matter if you fill it or not, the game decides when you trigger the swarm. Notice you can also trigger swarm mode from the picking bonus. I might be very well wrong about this tho, so I'm glad if someone can explain how it works.
There are also games where you collect x amount of stuff (there are many different versions and mechanics, but it is part of the games payout rtp) and then you either instantly trigger a bonus/feature or are eligible to trigger one. - there is chance you will never get the bonus (or all bonuses if there are multiple) or you might want to leave the game while you have x amount of stuff collected. If you don't leave game at optimal time you will lose some value unless you come back later. There are few games I could think of where there is no optimal time to quit. You will always leave some collected stuff in.
So TL DR: for fun YES why not, for winning bigger, better, increased rtp NOT REALLY