Now THAT I can help answer.
This is Fall 2014 ridership along the 5 in the northbound direction only. One would assume if you flipped the direction that the ons/offs within a given segment would flip (most people tend to take the same bus and use similar stops in reverse for whatever trip they make). In any case, segment 6 is the the downtown segment of the 5, and while you've got a lot of people getting on and off there, there's a decent amount of people using the bus solely outside of downtown as well. For example, ~1,600 people get on in the NB direction north of downtown vs 2,500 getting on NB within downtown. The only thing this doesn't tell us is how long people at any given segment are traveling - we can guess based on the ratio of ons/offs within each segment what % are taking long trips, but that could be 100% false.
Even still, this tells me the line is probably more on the "ridership is a function of the line's length" than the other way around. Lots of riders using it within the segments on either end of downtown, and the majority of the remainder seem to be getting off downtown as their end destination or transferring to another route. I'd still say that doesn't mean it makes a bad LRT candidate - it's exactly the behavior we see on the Green Line (and what many transit detractors don't understand about how it functions best not as a end-to-end vehicle, but serving many points in between).