This means that patterns such as:

if my_frame.index:
    # do something

with my_frame having a MultiIndex will never enter the if block. One can check that bool(my_frame.index) returns False. A single index (pandas.Index) will (correctly IMO) raise a ValueError warning that the truth value of an array is ambiguous. However, a MultiIndex fails silently.

This is such an awesome source of silent bugs that I'm surprised this hasn't come up before. Is there any reason why we want bool to return False every time for a MultiIndex?