The dev team has shown time and time again that they like to make older content relavent to brand new content. For example, 2.X beast tribes are a good source of Grade IV materia. Also they are one of the best ways to gain items for the relic weapon quest in 3.1 (Relics are basically long tedious grinds for a weapon as strong as you get for defeating the end game raids).

My advice is never to skip anything, but don't force yourself to do stuff you don't enjoy. Do your beast tribes, but if you start to get bored of them, skip them for a few days or weeks. Maybe just do 1 beast tribe quest every 3 days until you can be bothered to do more in a day.

Certain level 50 content isn't that popular at all. Like Odin Trial fight and Ultima Trial fight. However you can always ask friends to help you and even Novice Network to see if people are willing to help you.