Our wash stall is used daily.
It may not be used for bathing daily, but it is used to clean water buckets, feed tubs, its also used as an extra grooming stall, when the main grooming stall is occupied.
When I built my barn I knew one of my must haves was a wash stall. A place to bath horses year round if need be, and a place to be able to do washing and cleaning.
Whether its hosing off muddy/sweaty horses, hosing off muddy blankets, cold hosing a injured horse, washing and cleaning feed tubs, the wash stall is a God send. Would not replace it for anything....and I know my boarders agree w me on that one!
In winter it gets used less then spring/summer/fall for the horses themselves, but like I say it still gets used every day.
Just the other day we had a horse who got a small cut on their leg (nothing major) but of course it blew up like a balloon...cold hosing 15min/day made the swelling go down a whole lot faster then if we hadn't had that option. Can't imagine attempting that in -20 w no wash stall.