X spec being better does not mean Y spec is bad, even relatively like you say.
The top 3 healer specs (Disc, Hpal, Rsham) all have too much and that’s where the issue lies. Besides, the difference between the healer specs is much smaller than people are led to believe. In terms of HPS, every healer is very close. In terms of DPS, Mistweaver is pretty high up there. MW brings a lot of valuable things to the raid that the other specs don’t. Are paladins good at raid healing? No. Is Disc good at spot healing? No. Does shaman do any damage? No. Does MW do all of those things while also bringing the highest HPS cd in the game? Yes. Sure, DR’s are usually better but don’t sleep on the amount of healing that Yu’lon can do.
I go over this issue here: Mistweaver buff request - #107 by Nasella-kazzak
I really don’t feel like typing it out again, so give that a read if you have time.
The fact that MW is not as strong as those specs is literally irrelevant for 99% of the playerbase. In my guild we run a resto druid and MW for every fight. Do you think we have run into a single problem because we use those two “bad” classes? Do you think it’s somehow hindering us on fights where damage reduction is huge like Mythic Council?