A best friend need not come for help if s/he is incapable of doing so even when unmarried. Most of us will not break a friendship on that account. If it happens that person was not a true friend in the first place.
After marriage this instances of incapability should be expected to increase many folds.
However, marrying a best friend is still may not a good idea, if you are expecting that the friendship will last forever. It may not happen in many cases. There are just too many compromises and sacrifices to be made in a married life.
