Double payment on a 30 yr mortgage

[Marty Sheehan]

I ran a hypothetical mortgage payment on a calculator and found that when making double payments (one normal and one principle only) on a 30 yr mortgage you pay less in interest then single payments on a 15 yr.  I hear a lot of investors on here using a 15 yr mortgage as one of their criteria.  Can anyone who does this explain why they would use a 15 yr mortgage over making double payments on a 30 yr? Is it taxes, expenses, just simpler, etc.?