I do feel like merit is a popularity contest. It's totally worthless for Legendary members, just bragging rights yet you see Legendary members getting merited for everything.
It depends. If you are an active campaign participant, the merits are important even for legendary accounts. We have seen many campaigns where the earned merits in the last 120 days are taken into account. Nowadays, we are seeing campaigns where there is a merit-rank, enabling those with 3k or 4k of earned merits better pay rates than the rest of the participants. If you earn 2k and start thinking, now I don't care anymore and can relax, your position may be filled by someone who didn't relax. So, they have their uses. Let's call it that.
Someone who's capable of acquiring 2.000 merit in the first place is likely to earn more without any extravagant effort, firstly due to their status and secondly due to their knowledge. It's unlikely that someone with a legendary account with so much merit would sit back and relax and start posting gibberish. Creating constructive content isn't that difficult after all; it's possible if you're determined and patient enough.
However, I understand your point; this scenario is more probable with someone who ranks up from Full Member to Sr.Member or Hero, where you practically reach the peak of the signature campaign payment rates.