Anyone who have the interest of the forum at heart will agree to this especially if that individual have been around for at least two of those three eras (stages) you listed above. This system (merit) brought about more informative, educational and captivating topic. The merit system have achieve its initial purpose (that's bringing about quality discussion). We just have to build on it from here henceforth.
OP, congratulations on your soon to rank up. Thought you'll get there before @cabalism13 though you're almost there. It has been a journey, can remember our days of Full member like yesterday, another user I'm routing for is @r1s2g3. Took you guys long enough though congratulations.
Definitely, I think introducing the activity ranking system was an attempt to prevent account farming. If I remember correctly, signature campaigns started to pick up during this time, as well as the value, and publicity of Bitcoin. Therefore, the forum was subject to more posts, and more account registrations. I'll have a look at the data of this later on when I've got a bit more time, so this is all off my memory at the moment.
So the activity system was introduced to prevent users ranking up by spamming, and then potentially trying to scam. Back then we didn't have a trust system, and high ranks were considered more trustworthy, because it seems our society values higher, and longer term members than newer ones, which I think we as a forum have been proven wrong on this assumption multiple times.
The merit system was then finally introduced to put a significant end to low quality users, account farming, and spammers. I think we'll always have a problem with spammers, and low quality users, but at least these days they don't get rewarded for it. Again, we'll always have account farming, but for different reasons. The majority of account farmers are now those that register hundreds of accounts to bump, and advertise their services.