They may create drag... But at our level, and dare I say anyone elses, nowhere near even a considerable amount. Spinning a wheel alone will have much less inertia than spinning the wheel via going down the road, thus the friction due to bearing seals being negligible in comparison to say air resistance of a moving body (which squares up as you increase speed).
However, some people say that loose-balls (when set correctly) roll smoother. Myself personally, can not tell the difference, and I ride on both, all the time. The only thing I do know, is that it can be a pain if the cones loosen and every bit of grit will just be swallowed into the hubs. But this is on a Formula hub, my 105 hubs don't have issues with the cones coming loose. This may also be in part to my 3 dollar cone wrenches, but who knows. IMHO, it's really personal preference. If you like looseballs (because they spin for more time while off the road or roll smoother) then go with looseballs, if you like sealed cartridge bearings (because they stay grit free for longer, and don't require cone adjustments ever or what have you) then go with sealed bearings.
Really, the only absolute advantage that cartridge bearings have over loose-balls, is that when that cartridge goes out, you simply throw in another. The press fit will stay, and if not, is easily re-obtainable. Whereas with loose-balls, assuming you keep your hubs/wheelset long enough, if those races go too long without maintenance, the hubs are screwed (to my understanding, replacement cups are pretty rare or expensive).