I think it depends on their form of retail therapy. My most recent "luxury" purchases (aside from vet bills and car repairs) have been a brand new vacuum cleaner to replace my really sucky old one, and a dremel so that I can complete four projects without ruining my hands. Both purchases gave me a retail-therapy high - but being able to quickly and easily complete those projects and put everything away, and finally having a clean house, are both giving me an even longer-lasting high, when I can walk around without feeling the grime in the carpet, and I can walk into the dining area and not immediately be confronted by a table of "oh, yeah, I really need to finish those, shit".

Did I absolutely need the vacuum or the dremel? Nope, their purchase was retail therapy, I definitely did it for the high. But I justified the purchases by saying they'd be good for me - and they have been.