According to Practical English Usage, "We use used to, not would, to talk about regular and important habitual behaviour.
Robert used to play a lot of football. (NOT Robert would play ...)
I used to smoke. (NOT I would smoke ...)"
I don't understand why used to can't be replaced with would in the examples above since they express the idea of repeated actions to which both used to and would can refer to.
By the way, what does important habitual behaviour exactly mean?