Why is prepaying a mortgage a good thing with today's mortgage rates? Seems like a dumb thing to do, just take the excess money and invest it - all you need to make is more than ~4% (if not less).
Here is another way to look at this:
Prepaying a (presumably fixed rate) mortgage converts a guaranteed CASH LIQUID asset into non-guaranteed, illiquid HOME EQUITY.
Now I know that when home prices are rising and banks are easily giving HELOCs, many people might not realize the risks involved. But remember that HELOCs can be frozen, shut down, or simply not offerred at any time the banks don't want to lend. Also, while we've seen an outstanding increase in home values (especially in BKLN and LKWD), prices don't always go up (they are possibly a little down from their peak). And here's something many people miss - prepaying a mortgage is irrelevant to your net worth, I actually know people that were kicking themselves for not having larger mortgage balances during the Making Home Affordable loan modification days.
Also, don't forget tax ramifications.