Its probably not that big of a deal. I have had my current STI going on 3 years now...and haven't pressed the I/C sprayer once. And yes, it gets quite hot up here in the Philly area as well.
Only time I ever used it is when I was hot lapping at the drag strip in my first STI years back. I'd actually put ice water and a rubbing alcohol mixture into the sprayer tank and would continually hit the sprayer while staging. Outside of a situation like that, its probably not worth the money IMO.