My parents' cape house had knee walls along the front of the roofline. The spaces they created were horrible. They were too small to easily use for storage; they were nothing but a conduit for drafts, bugs, fire and rodents. Fortunately for us, we never had problems with the latter two, but if, say, a fire had started in one of the bedrooms, it would have engulfed the whole second 3/4 story because of those spaces. There was even a tunnel over the stairs connecting the front crawlspaces to the one above the bedroom ceilings, as if an arsonist planned it that way.
If I could have designed that house, I would have lined the whole triangular area with drywall, then fill in the peaks with cabinets and shelving units to make use of the space; the peak above the bedroom ceilings could have been used for finished, insulated sleeping lofts. That would have made it a much safer, more efficient, less bug-prone house.