mom's morning out is usually for 2 year olds, primarily for socialization, IMO...
preschool around here starts at age 3 (potty trained) but only at a private/church based/etc. type school. Our public school doesn't start until Kindergarten, which is a FULL day program.
I started dd in preschool at 3 - it was 2 days a week, from 9:30-12. This year she is in pre-K (the same church based private school) from 9:30-12, M-F Next year she'll start public K, which is from 8:30-3 every day. Unfortunately, that is what it is... I can't change the way our county runs their public K program.
For me, it wasn't so much the 'academic' progress that I was worried about. I'm a firm believer that kids will learn in their own time and that pushing them only causes strife down the road (for my little one it does anyway! LOL

) she *is* an only child for now, and I wanted her to
A: have more socialization opportunities with like-minded kids/moms. Her school is actually pretty AP - they use gentle discipline as much as possible, very calm, nurturing environment. and
B: give her a chance to begin to adapt to the 'school routine' that is going to be inevitable down the pike... I thought the progression from 2 days, to 5 half days, to 5 full days was as 'gentle' as I could make it. My daughter would NOT have done well to go from a daycare/home environment, no matter how scheduled, to a full day school environment. I just know that would be bad for her personality. So far, she has eased into it ok. And the beauty with pre-K this year is that yes, we do pay for it every month, but because it is not required by our county (they only provide it for economically disadvantaged/learning disabled students) if she REALLY doesn't want to go to school one day, we'll sit down and talk about it beforehand. she's stayed home a few days already simply because her little circuits were just overloaded. Now, that isn't an option next year - but I feel confident that as the year goes on, she'll progress enough in that direction to handle it.
the only other option, besides continuing a MUCH more expensive private education route (catholic or montessori - the program she is in now does not do K) is to homeschool, which would not work for either of us. I've considered it many times, and it just isn't going to be an option unless it becomes necessary due to other circumstances.
To be quite honest, I wouldn't use daycare unless I *needed* daycare - i.e. if I was working. There are lots of programs you can get your toddler/preschooler into for socialization type stuff if you don't want to go the 'school' route... my dd never went to daycare because I was a SAHM, but I did decide to do the school route with her since it seemed like a natural progression - and not too rigid.