i really think this is a personal thing and lots of moms will have different thoughts on this depending on the child.
for us, DS and DD didn't need preschool that early. as you said, i was here and able to take care of them (and frankly here it is expensive and where i am at home it didn't seem a necessary expense). looking at them i could see they interacted well with other children, they had a good measurable set of 2 yr old skills and were happy most of the time. we decided this year to enroll them because they seemed ready for something more - the days were harder to keep them entertained and they were asking to go to school (something they heard about from older friends). they are excelling at 2 mornings a week and could probably do more, but with that i am comfortable having them with me the rest of the time. i figure there is plenty of tiem for school later on!!