Is the Thanksgiving Banner worth it?

Up to you. I personally would save for Merlin since strong support > everything else in terms of return on investment, but really just go for Scathach if you want her. Gil and Raikou should both be on rate-up again around mid-2019 if you can wait, based on JP's schedule Scathach will be available again for new year 2020 (or thanksgiving 2019 like she is now idk).

Merlin is available again next year too, though bear in mind that it'll be the last time he'll be on rate-up for free for a while.

I think it's worth it if you like any of those servants.

Strictly gameplay---> None of those servants are more useful than Merlin. For farming he's like Helena with higher damage potential, for hard content he's the craziest motherfucker ever.

Being logical ---> This game is easy and you don't need Merlin to clear anything. So you can easily skip him if you don't like him.

If you're f2p you might want to seriously consider saving everything for Merlin under the assumption you'll have the worst luck (because statistically you probably will) and like waver, he's a support you're at a disadvantage at when he's not on your team. Yes, you could always borrow one but its nice to have your own with the option of a dps support choice.

You can always grab these thanksgiving servants again on a gssr banner or other banners next year but Merlin doesn't come around a whole lot on the JP side so you'll want to definitely grab him at new years if you can afford it.