I'm a little dismayed by some of the snarky comments here.
I think there's no single right way to parent a child. Each parent or couple decides what's best. I think it's a huge leap to assume that you are taking the spots needed by some single working mother or that you are being neglectful! By that logic, we would all homeschool all the time, never use preschool, and we would never go out to dinner with our spouses, so we could be with our kids 24/7.
If you need to get stuff done, and you have a supportive program that is nurturing and enjoyable, if your younger kids are learning how to deal with a longer school day as well as transitions, if you can afford it, what's the problem?
I put my child in preschool 2 days a week to start, and moved up progressively to 3 days and then 3 days plus 1 lunch, then pre-K, then pre-K plus 2 lunches/extended day. It wasn't for academics - it was for socialization, enrichment, cultural activities and to give me time to get stuff done and have a life that wasn't based on a child 24/7.
I think you feel guilty because you are listening to someone else tell you they wouldn't do that. But plenty of other moms ARE doing it, so you aren't looking to the right people for support if you still feel bothered.
The other thing is, if you are spending too much time on errands and chores and stuff you SHOULD do, maybe you aren't getting enough time for just YOU. Not caring for yourself can spill over into feeling you aren't worthy, and than can make you feel like you aren't perfect at everything you're supposed to be perfect at. I hope you use SOME of the time when they are in school to read a book, go to the gym, meditate, knit, volunteer, or do whatever it is that fulfills and restores you. That's critical for your own rejuvenation, and will make you a better mom when you are with your kids.
Just try to USE the time and not measure it in terms of how many toilets you scrubbed or how much recycling you did (because there is always stuff left unfinished), but in terms of whether you did something to better the world and better yourself.