If you have a sub-prime loan, (if those still exist in your jurisdiction) I guess you should have to worry about liquidity.
There are those of us who want to get out of debt, and there are those who want to ride out the mortgage to its maximum length because the market does give better returns.
We all agree, however, that you should prioritise your tax-advantaged accounts ahead of low-interest mortgages. The payback on those (traditional 401k in the US? RRSP in Canada) is so large you just have to take advantage of them, no matter what your feelings are about mortgages and debt.
If you're maxing out those accounts, then you can decide how you want to invest the rest of your money: a 3.5% guaranteed mortgage pay-off or a 7.x% average stock market (knowing that, in the U.S., your mortgage interest is tax deductible)
Toque.