how do you "simply put a voltage"?, it's the same as a digital pot, you put vcc, vref and output so it's almost the same for both devices, you simply put the voltage.
i use 8 bit digital pot for a 0-5V precision adjust of qusi-static output}(maybe changes once every minute), i find it offers the same performance as an 8bit DAC but much simpler(both offer 256 steps from 0 to 5v), as almost any serial 8-bit dac is multiple output and SMD (which i don't want, and haven't found a single DIP I2C serial 8bit simple voltage output DAC, heck, it's hard to even find a serial 8bit dac at all, the 8bit dacs normally found are +10yr designs like the DAC083 series from National that use parallel bus, which is a no-go).
pricewise i'd say they're the same.
Maybe if i where to need 12bit or differential output or current output or even simultaneous multiples outputs or high speed or if there where some problem with the variable output impedance of the pot output(easily solvable with a opamp) then a DAC would be the only choice(and some dacs even require external buffers, so kind of moot), but for semi-static 8bit 0-5v single ended output, it's the same(and i think digital pot commands are simpler than a DACs)
at the end, everyone uses what they feel more comfortable/simpler/best for their application based on the data they have