It depends on the parent, but it also depends on the "preschool." This comes back around to the same arguments that are used about home schooling. Some preschools really do help teach some fundamental things, and with parents having to work, there is an unfortunate limit on how much time a parent has to spend with a child. My mother taught for years and years at schools that got kids started on their reading and writing (yes, in preschool) and foreign language skills (only at some schools, and only after gaining parental consent and input as to what languages to speak, etc.). The schools even got into some pretty interesting cultural backgrounds, with parents coming in and sharing their stories, foods, etc. It was more of a community.
There are too many schools that are babysitting services. You drop your child off, they go running around with whatever insane little monsters are already there, and some of those children are literally there all day. There was one place open near where I used to live that was open until MIDNIGHT. Please tell me what a four year old is doing outside the home or a family/friend's home at midnight? Ever? It's ridiculous. Oh I know: mommy has to work the late shift at wherever. I'm sure they also have early shifts. I'm sure there are other places to work. I'm sure you have family members or friends or in-house people who can watch your child sleep.
There are places that do not clean, places that teach children to "never ever ever" defend themselves, to speak quietly at all times, to never run, and to be kind to strangers. Yes, you heard the last one correctly. Remember when you were growing up and they said to be wary of stranger danger? No more! We're supposed to be kind to them. Listen to authority, as well.
My mother stopped teaching because she was in her 50's and was being overrun with the new schooling. She had to have someone with her at all times, even when assisting a child who'd soiled themselves, because the worry was if a teacher was alone with a student, especially in a bathroom, the child might say something to the parent, and the parent might sue. Things like that happen. "So and so changed my underwear" turns into inappropriate touches. It's a wonder the infants (yes, infants were brought into the schools by the end of it) and special needs (yes) children didn't also protest at having their diapers changed. It is, after all, highly inappropriate touching!
A woman in her 50's who is used to looking after small children shouldn't have to be forced into lugging around 8 year old wheelchair-bound children with no motor or bowel control. The school at that time, though, received an awful lot more money by having those children there. The playgrounds were changed. Mobile games like Tag or Hide-n-Seek were discouraged, because not everyone could take part equally. I Spy was encouraged, which is great and all, but it isn't everyone's cup of tea.
I guess the point is: Can you provide time and instruction for your children? Will they be supervised appropriately? Will whoever is supervising them follow your wishes (in or outside the home)? Will they see a bit of the world and make a friend or two?
P.S. - If you are going to homeschool (or groupschool, where parents pool their time and knowledge), please make sure someone in the house knows CPR

Lots of people overlook this, and depending on 9-1-1 isn't the wisest. Childcare professionals generally get certified, and classes are cheap/free.