Okay, I see the edit in your original post - so this is less about SAHP v. career, and more about those who want to do both.
I already answered your survey, but I want to add some other thoughts. I think being a SAHP to an infant is really different from being a SAHP to a preschooler or kindergartener. IDK about others, but I felt that caring for an infant was basically a full time job- there was no time for a part time job, at least no more time than one with another full time job would have. Of course, a lot of this was based on parenting choices- 100% breastfeeding (no pumping), ECing, cloth diapering, no pacifying baby gear (swings, etc). Preschoolers- if they are in school part time, than part time work becomes more of an option.
I definitely think flexible schedules, job sharing, telecommuting, part-time work, etc. are not utilized well in most work places. I agree it is a waste of resources that employers don't take advantage of SAHP that would do great work because they can't be in a seat M-F, 8-5.
I think there are some industries that are more open to this kind of flexibility. I work in accounting, and when I was in public, I knew several parents that worked part time on the tax side. Much easier to do on the tax side than the audit side, I think. And there are several local/regional firms that specialize in providing outsourced accounting professionals- not contract work, but you are an employee of a firm that, say, specializes in controller services for mid-size non-profits. From what I hear, the pay is comparable (on a hourly basis) to full time opportunities.
I also have a friend that is an auto-cad tech, I think she works for an architecture firm, and it's a small organization. They are really flexible with her. She is a SAHP with options to work part time. I wish more organizations were like hers!