“The Offspring” makes me cry.
That is all.
Because the episode was written by the usual gang of idiots at Voyager, who never watched any episodes from the previous three series, and have no interest in keeping any sort of continuity…
While I don’t disagree with Barbarian’s answer, one should note that the Doctor’s sentience was never fully accepted as Data’s was. The Doctor (and even if not him, the other MK I’s, which are much less developed because of less bredth and depth of experience) may actually only be a non-sentient machine doing a very good impression of an intelligent creature, sort of a chatbot to the nth power.
As for the use of them, the thinking seemed to be that they had all these damn MK I’s lying around, they should use 'em for something, and if they were already going to be used as drudges, why spend resources to change their appearance? (Perhaps they had specialized hardware which made it more cost-efficient to use them elsewhere than to use that computing power for something else.)
–Cliffy
The idea of a bunch of Datas came up in the excellent season 5 episode “Conundrum”, when they briefly hypothesized that each Starfleet ship might have an artificial being.