Keith Chrzan provided a more scientific answer than I will here.
From a practical standpoint, respondents can often be quite repetitious in their answers to MBC questionnaires. Not surprisingly, they want very similar combinations of things across menu tasks, and often even substantial price changes won't dissuade them much from their choice patterns.
A recent simulation study by Neuerburg at our last conference suggests that results don't improve that much for MBC under HB modeling once respondents answer 5 or 6 choice tasks. I tend to favor about 6 to 8 MBC tasks for my work so far. But, MBC encompasses so many kinds of designs and situations, that I suspect both Neuerburg and my opinions won't hold for all situations.