I disagree, because Waver is universal sure, but he isn't always needed. Always useful yeah, but sometimes being useful isn't enough. Merlin gives the party wide invunerability, a party wide boost to NP, and his NP heals, and charges up your NP faster.
Or to put it another way, I have a Waver. I could've taken a support Waver. Neither or both would've gotten me through the Gudaguda challenge quest. I needed Merlin to beat that. Waver doesn't really cover Merlin's role of buffing defenses, but Merlin kinda covers Waver's of NP generation between his skills and his NP.
Sadly, it isn't looking good for my chances of getting him. I got two Medusas and two Tomoe Gozens but no First Hassan. I can get another ten draw before the banner is over, but that'll be my last chance.