Isn't it just our expectations of what being a parent is about? This wouldn't have been uncommon in Victorian times (when help was cheap and incomes much more unequal) or indeed in developing countries now - have a friend who was born in Ethiopia in a nice middle class family and they had nanny, cook, gardener, housekeeper, the works - and she has a very good relationship with her parents. I reckon if you can afford it why not, if you can cope with the guilt! I already have a cleaner and a childminder though so I'm some way down the road to damnation anyway.
If I were not working, I wouldn't have a full-time nanny but the though of having an au pair to help out + do all the drudge stuff seems quite appealing. And of course on the proviso that they were lovely and loved children, so that I wouldn't feel guilty I was damaging their development a la Mr Biddulph.
I can think of lots of things I'd do with my time if i wasn't working or having to look after the kids - like recover from this horrible coughing bug without getting out of bed! It's just the idea of having any time you spend with your kids being voluntary, not compulsory - imagine, sleeping as much you wanted (can you tell sleep is an isue at the moment)! You could spend quality time with one while au pair looked after the other, or head off on outings with spare pair of hands when DS ran off for the umpteenth time. And would make things like meal times, weekends, much more relaxed.
BTW nailpolish, I go to work even though in theory we could manage on DH's income (and with childcare for two kids hardly worth it) because (1) tbh I like having the break and thinking about non-kid related things (2) I'm not sure I could go back to work with a really big break - and don't think I could rely on DH to always be the provider or indeed to be around (cross fingers and all that). So even though I don't strictly need to be a WOOHM I am albeit part-time. In fact part way through maternity leave when I was in NZ staying with my parents, with DS a particularly challenging 3-yr old and DD only 3mths so no way of leaving her, and DH back here working, I actually felt as if I was going mad and deteriorating as a parent - too shouty and intolerant.