Totally unnecessary to provide UV but I've also used reptisun 18'' bulbs (never expose albinos to UV including sunlight). It does seem to have some behavioral influence such as more activity, increased appetite, etc. They just seem more lively. However, I get the same result from any bulb that provides a naturalistic color balance and intensity, even if it doesn't put out UV. The more closely you can approximate natural daylight spectrum the better I think, but it doesn't necessarily have to put out UV. Snakes can get all the vitamin D they need from their food, unlike many lizards which can only synthesize it.

Most, if not all snakes are like humans in that a little UV exposure does sythesize vitamin D, (about 10 minutes of direct sun per week will synthesize all the D we need) but as long as we get some from our food, we can live without explosure to UV. Many lizards cannot live without UV but that's not the case with snakes.

On a side note, I noticed that in the longest days of the year, garters here tend to avoid the midday sun when the UV index is highest, even if it's a very cool day. From about 11:30am to 5pm they just vanish. Doesn't seem to be tied to the temperature since we often have the highest temperatures after 5pm and they still come out to bask. Seems they're really just avoiding the high UV. Of course, in fall, winter, or early spring, they don't avoid midday sun and I think that's because the UV index is much lower.