In most states it's illegal to have a rear facing infant seat in the front of a vehicle with air bags. Other than that, they generally advise (not require) for kids to sit in the back. My kids have taken turns in the front since an early age because at the time we had three kids and a 2-door hatchback. Three car seats (or two car seats and a booster) would not fit on the rear seat, so we had to have someone up front.
To me, it seems safer to have a young child in the front seat where you can help them with things (sippy cups, reading material, loose socks) than to be driving with one hand and reaching around in the back with the other hand.
I can't tell you how many times I've been behind a car that's swerving all over the road, and when I pass it I see a mom in the front with her eyes glued to her mirror and two or three screaming kids in the back.