Why?
Seriously, why? If you know you'll be moving in 10-15 years then why not just buy a home you think you'll need for that time? Maybe you won't have 5 kids, but you can always use extra bedrooms a lot easier than you can fit 4 kids into a 2 bedroom house you can't sell cause the market tanked in your area.
My husband and I are looking into home ownership so the question came up with no real answer yet