This doesn't have to be solved by allowing the player to select a difficulty for a fight. The way it stands now, there are very few endgame events, one of them being quite challenging. The way that Yoshi-P envisioned endgame content throughout this string of updates is the separation of hardcore from casual gamers. I think he said something along the lines of the hardcore players will beat it first, derive a strategy and over the course of a couple months the casual gamers will catch up and beat it too. I can already see this happening on Lindblum, more and more people are running around with Ifrit weapons, and something that used to appear hard, we now beat every fight like its nothing. That is the way that Yoshi-P envisioned the game's content, and I'd say it's doing its job.
Once more content is added, say similar to the amount of content in FFXI, I'm sure that the casual players would gravitate towards the easier events to begin with, and be able to work their way up. There is no need to have a tiered difficulty system in place when there is a large range of events to participate in. If you and your linkshell can't complete something you just have to wait until either you're better or you have the time to spend practicing it. That's just the way it is in ALL games, it's why they're challenging and why they're fun. Beating something you set on easy mode would be boring as hell, I think it's a terrible idea.
Edit: Oh and anyone who disagrees with you is not a troll, fyi.