Seems to me, 2 crucial items keep getting left out of most of these analyses:
1) Taxes - if you're in a high tax bracket, the value of paying off a mortgage early may be less than it is for someone in a low tax bracket. Essentially, if two people have the same mortgage but one person's income is so low that they pay 0% in income taxes, and the other person has a high income and gets a 35% tax break on the mortgage interest, the low-income person has a better benefit from paying the loan off early. For a mortgage payment of $1,000 a month with 900 of that as interest, the low-income person pays $1,000, but the high-income person only pays $700 (because of their tax break).
2) Risk - many people are comparing pre-paying the mortgage versus investing in the stock market. This is a false analogy as the risks are far different. Pre-paying the mortgage is a low-risk, guaranteed rate of return. If I prepay a 4% mortgage, I am guaranteed to save 4% interest. The stock market can go up or down (historical averages notwithstanding, as past performance is no guarantee of future performance). The true comparison would be between the mortgage interest rate, and what you can currently get from safe fixed investments like high-grade bonds or CDs (or possibly one might include dividend paying stocks in super-safe giants like Disney, if you only counted dividend income and not stock appreciation).
Also, I think everyone should at least try to pay down their mortgage to below 70% to protect them against the possibility of being trapped in a location by an underwater mortgage, unless you have enough assets already to ride out that scenario. A lot of people suffered in the last downturn because they lost jobs and their stock investments dropped at the same time - their houses were underwater and they had to move to get a new job. This financial meltdown could have been avoided by a paid-off home.