Because in my opinion the become a crutch more as anything else.
I can completely agree that they can do more harm than good for a young horse and "become a crutch" but for an older horse, they can be really invaluable.
It's been proven that, as they age, horses, like humans, loose the ability to regulate their internal temperature as effectively as they were able to at a younger age. They have a harder time keeping themselves warm in the winter and a harder time keeping themselves cool in the summer, it's just the way it is.
In my opinion, it's a kindness to blanket an older horse that's showing signs of becoming unable to regulate his/her own body temperature.
I like to think of blanketing an older horse as thanking him/her for all the years of enjoyment he/she has given me. I want someone looking out for me in my golden years so I figure I'll give that same respect to a horse in their golden years.
As an example, my mare is turning 27 this year. She's in good weight, sassy as all get out, energetic, etc and I blanket her. Why?
Because that sassiness, that energy, and quite a bit of that weight, disappears asap if I leave her blanket off for a week of 35*F (or colder) at night weather even with her full "wooly mammoth" winter coat. I could give her as much hay as I want during that time but if she's blanket-less, that hay doesn't seem to matter.
She used to be not like that but as she's gotten older, she's gotten a little bit more needy in that area of things. It can be a hassle to keep her blanketed right but seeing her thriving, at her age especially, is worth it.
Just my thoughts.
Blankets for younger horse=not a huge fan, blankets for old man and old woman horses=huge fan.