honestly, as long as options are spelled out, no, you don't have to provide it. is it nice, sure! a must, no. some may disagree, but, i've been to out of town locations for wedding and events and i don't "expect" people to get me from point a to point b.
now, having said that, i would definitely provide all the info possible to folks ahead of time (which they may or may not take in) and i would make every effort to make my planning conducive to people getting around easily - like locations that are close-ish and times amenable to getting around. and, as i think you seem to understand, there are only certain events in which transportation is "mandatory" per DFTW.
we got married at the WP and the reception followed at the GF. we ended up using a limo for the minimum time to take us to and fro and then to loop folks back and forth if they met the limo at where we told them. lots of folks did have rentals anyway. i'm not aware of any problems with what we offered and actually i was glad in the end that we had the limo, our ride over to the GF after pictures was a nice exciting and quiet private time for us.
happy planning!