I really doubt that snakes benefit from UVA or UVB like lizards or turtles do. I'd also be concerned about sunburn with regards to snows and amels.
However I use a heatlamp in conjunction with a UTH because, like SkyChimp says, I find it it creates a concentrated basking spot and also ups the ambient temperature nicely. I use a 40w reflector bulb above the tank, angled over one side to create a gradient. It's also on a timer so it turns off at night and with the timer I can adjust the photoperiod throughout the year.
As with all heating devices, do your homework and experiment with it in different positions, different hieghts etc, before you put the snake in the tank. I also think that if a UTH heats the tank fine for you, and the tank isn't locked away in a dark room, then a heatlamp is probably unnecessary - the simpler your heating the arrangement is, the easier it will be to moniter. There's no point spending extra money and time on something you don't need.