Jimp,
Many people can pay more principle on their mortgages each month but choose not to, and instead direct that available monies towards liquid savings, or other investments that yield a higher return. For them, *IF* refinancing is cheap (or $0) and the liklihood of staying in the home for the loan duration is high, refi'ing makes sense as it can mean a lower payment each month and lower interest paid over the remaining term of the loan.
We refi'ed our current home twice. Lowered the monthly payment AND term each time, as well as the total interest to be paid on the loan. The first refi had closing costs, the second did not.
I've heard people toss around 2% as a guide; any savings at a 2% rate drop (or higher) is worth the refi; below it is not. But I don't think there is a one-size fits all.
Amortization tables/spreadsheets are easy to come by. The analysis of the pros/cons to refi or not are easily calculated. However, their are uncertainities in the scenarios compared that just can't be known.
TJR