Actually, they'll probably get more UVB rays from an hour in the sun on a bright day than 12 hours of an artificial UVB light (although the mercury vapor bulbs do give out significantly more UVB rays than regular UVB lights). The sun emits much stronger UVB rays than a bulb.
If you can take your turtles out several times a week for a few hours each time in the summer, this should suffice for their UVB needs. The problem is doing it on a consistent basis--the weather may not cooperate and you might not have the time to do it regularly (and even if you could, you won't be able to do it for 6 months unless you're in a really warm climate) . Getting a UVB light would be best.
A turtle doesn't know if the light is a UVB one or not, so the issue is not whether they'd be more comfortable with one; they'll happily bask under a light that produces no UVB rays at all as too many turtle keepers allow them to do (they're after the heat). But in time, without sufficient Vit D3 (which is what the UVB are converted into), they won't be able to absorb the calcium in their diet, and are at risk of developing metabolic bone disease (aka soft shell disease). The disease is a major killer of young aquatic turtles. It would seem to make sense (and possibly avert future health problems) to just get a UVB light to help prevent this from happening.

"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-