Currently, about half of what I own, weight-wise, is in storage. it's free to me (paid by DH's employer). When we are reunited with that stuff, whenever that might be, I'm sure at least half of it will be donated, trashed, or sold, as living without it has been a great exercise for us.
But I had no choice in selecting the unit, and I've no idea if it is climate controlled. And I worry greatly about the effects of humidity. It's not the temps; it's the moisture. A few things went in there by mistake, in all the chaos, like my wedding album. Thankfully, the stuff was picked up in a fairly dry climate and likely stayed in the area.
So while generally I think storage in general is mostly silly if one is paying for it, I can see why A/C would seem like a good idea if one was set on doing storage.