The only reason I bought Spa Day was because it kept showing up in gallery content downloads and I got tired of builds with so many things I had to fix.
Now that I have it, that still seems to be the only value I got out of it. Most of the build items in it I don't even care for - so it's mostly just there so I don't get an error message on grabbing something from the gallery, can then go in, and see the bland stuff rather than an empty spot, and manually know what to replace.
If the game provided some kind of market for "we removed a chair from right here" I would not have bought Spa Day...
Dine out on the other hand added a lot of new play for me and I use the build and CAS items from it all the time.
Restaurants make sense for me as a place for Sims to go often. Spas seem about as 'normal' of a visit spot as having a Pack for the DMV. Except, at least everybody has to go to the DMV a few times now and then - Spas are just too obscure.
Still not sure what the Wellness skill even does for my sims.
I've been trying to add yoga mats and meditation things to various builds - but they're just not something I relate to so they always feel like oddball additions and the yoga mat and meditation seat "footprints" in build mode are both about 300x bigger than the actual object, so unless you turn on the bb.moveobjects cheat you can fit about 1 every 2 lots and that's it.