Is it worth using quarts to NP2 Merlin?

Unlike the Merlin we met in the Singularity (I'll not mention which for the sake of anybody that has not played them all yet), the Merlin we can summon does not have a direct heal on his NP, only a heal-per-turn effect. The heal-per-turn scales a little more strongly, though.

Going from NP1 to NP2 changes the heal-per-turn from 1,000 per turn (5,000 total) to 1,500 per turn (7,500 total). There is no other effect of a higher NP level - the NP charge per turn is a static 5%, and the stars-per-turn is still based on Overcharge percentage (which you're unlikely to use, as a single 200% NP generates the same stars per turn as 2 100% NPs, except it only applies the heal and NP charge effects once, for an overall loss of a lot of healing and NP gauge in exchange for an extra attack on a later turn).

In my opinion, having used a LOT of SQ to get him, it's only worth it if you get a second with whatever spare SQ you have on you, and you don't want any of the upcoming servants. I think that's unlikely.

Think of Merlin as two halves - his skills are all about making others stronger, and his NP is about sustaining his allies. By increasing his NP level, you make one aspect of his sustain stronger by a mostly-negligible amount (if you need to heal a lot, simply get his NP cycling itself and unload lots of them to keep the buff up, there's rarely a time where an extra 500turn/2500 total will save you while not cycling his NP), and you likely use up a lot of SQ.

I just don't feel that's a good trade.

Not worth it as Tweets has explained so well. Unless of course he's your fave, or you allocated all those SQ for him anyway, and you have no upcoming servants you want. I have also thought about trying to NP2 him, then I remind myself that I really want Musashi and especially King Hassan.

Please, always remember: There is NO GUARANTEE that you actually will get him a second time. You could be using up all your long saved sq and end up with nothing while there are 3 more (I count Okita as well for now) great Limited Servants are coming really soon.
Please think about it again and if he is really that.

Im just curious. I just started around 3 months ago. How many quaryz did u save since the game started?

Not worth. All you get with NP2 Merlin is a stronger heal and potential for more stars per turn with overcharge. That isn't bad as far as support servants go, but saving the quartz for other servants is much better.