Hi Paul,
I am probably the biggest fan of the spa blankets. I think they help hold in the heat and protect your cover from becoming waterlogged or damaged by chemicals. So I look at them as the first line of defense for heat retention and preservation of the cover. They do have a life though, and I think I have been through 3 in 5 years. Eventually, the blanket breaks down so you can weigh the cost of multiple blanket purchases vs. what I am telling you that I believe.
Another issue is removing the blanket. I haven't really had a problem with that at all, I just flip it over my cover that is up on the lift. I guess imagine how you would remove it in that cold weather and you can determine if you want that added job. For some folks, that is the deal breaker..
Good luck