This is a timely topic for us, as our daughter is 3 yrs old, DH is a SAHD with her, she currently has not attended preschool, but we'll be finding one for her for next year.
Really, I don't think preschool for privileged kids is in any way necessary, or really has any impact either way. I was actually planning on DD not going as the default choice. However, DD just loves activities, getting out of the house,and is very high energy, so not only do I think she'll love it, but frankly, DH and I won't mind getting her out of the house at least a few mornings a week.
Our challenge will be finding a program that meets our needs. There is a Montessori program, but at almost $500 a month, I can't stomach it. The other options are all church programs. Which are super inexpensive, but as atheists, we're really going to have to check out exactly what's involved.