If you’re serious about rolling for Merlin, then this is your last chance for the foreseeable future. You don’t explicitly need him – there are always other comps and teams which can pull you through challenge quests – but Double Merlin with its stacked Avalon regen and double invincibility is about as easy as it gets for current NA content.
That said, I ran the quartz spreadsheets, and it looks like we have roughly 380 quartz coming to us from now 'till the end of the Merlin banner. That’s enough for a decent shot at Merlin even if you do blow all of your current quartz. There’s one major problem with the idea that you can save for the rerun next summer, though – Skadi’s banner follows soon afterward, and like Merlin, she has no foreseeable future banners beyond that. I imagine if you’re seriously considering Merlin for gameplay advantages, Skadi will strongly appeal to you as well. But as always, pull whoever you like!