It depends very much on the boat, but especially, on the sail areas involved. As sail areas get bigger, hand cranking the winches quickly becomes very difficult. Furthermore, it requires time and a lot of energy grinding, which a short handed crew may not have to spare, with a lot of other things on their mind.
We have four powered winches on my boat -- two large primary sheet winches (ST64's), and two coachroof winches (an ST44 and an ST42) which are used for -- mainsheet, vang, staysail sheet, mainsail furling
, topping lift
Trying to manage my boat without them, or even without two of them, would involve a major reallocation of crew resource and would really almost prevent using the boat short handed. The yankee sheets
are subject to such forces that hand grinding them with one person would be so slow, that the life of the sail would be seriously compromised from flogging while you're trying to grind it in.
So on my boat at least, worth their weight in gold. I wouldn't mind a couple more, actually. We have 10 winches and there are never enough.