First, just because it is beginning to cool down doesn't mean that the glare from the sun lessens. Window tint does a fantastic job of shielding your eyes from potentially blinding glares - whether from the sun or water reflection of some other lights. This means safety.
Second, those sun rays carry UV rays which damage your vehicle's interior. Tinted window can block up to 90% of those harmful rays and preserve the interior of your car.
Also, tinting your car helps increase privacy inside your vehicle as well as helps hold the glass together should the glass windows ever break. Again, tinting your windows increases safety for you and your passengers.
Most likely, you will be able to save money by having your car tinted in the fall and winter months as the installers are less busy and more eager to give customers a better deal.