oh yah it is a bit meh especially when your not fond of bunny. but it is going to keep popping up till you do it anyway so may as well do it. what it is supposed to show you is how much more power you can get out of the limited power it gives.
its a moot issue for players familiar with warframe but warframe was known for just handing you the system saying stuff goes here and thats it so its a little better.... just a little

this post seems a bit...meh

Bunny is pretty good. I'd put 4 cats onto Ultimate Bunny, sure.
That I get two back is a bonus.

Mainly though, I'd complete the stories if you are interested in the character.

The character stories are the best story content this game has to offer. Still not great but kinda nice.

Also you get a chest cosmetic for some of them.

But haven't done the ultimate bunny quest with the catalysts.. that is not a character story quest but more of a tutorial quest

If you're actually into the entire lore about descendants then it's very much worth it. I don't quite remember what the rewards were anymore as I finished them by now... I think it was catalysts or something, which is always useful. Can never have too many.