I know Merlin is THE servant to have, but I already have Waver and Tamamo as support Casters. Would it still be worth getting him or should those two be enough?
Well support system means you'll be able to borrow someone elses so its not essential to those who have Tamamo and Waver I think there are a few situations double Merlin is good though much like double Waver has its uses so all it all he'd be nice but not essential imo.
Everyone and their mom will roll for him and put him on their support list so you can borrow him whenever you need to. If you like him then go for it, if you really want to experience the power of double Merlin, then go for it... but otherwise no.
It really depends on your roster of servants and preferred team compositions. Waver is for general purposes, Tamamo is for arts team and Merlin is for buster crit team. I also have Tamamo and Waver but will roll for Merlin because having another support doesn't hurt (besides quartz).
All supports have different uses, and Merlin is pretty much good in a lot of team compositions. He has a party-wide invincibility as a skill, an attack up buff with NP charge and a massive boost for buster-oriented servants, So seeing his skill set, he can be a very useful servant to have.
But since you already have Waver, you don't really NEED Merlin as Waver is better as a generalist support IMO (thanks to his Def buff which lasts 3 turns).
Merlin is nice to have if you have the servants that can make use of his skills, mainly buster and crit-damaging servants like Jalter and Gilgamesh.
