I don't really do many structured intervals. Rather, I identify certain days as intensity days and make sure to drill it on the climbs on those days. Most rides around here average over 100' elevation gain/mile, and there isn't a single loop I do that isn't at least 75'/mile, so there are plenty of opportunities to go hard on climbs.
The other way that I get my intensity days in, is through actual racing. I'm usually racing XC mtb at least once a week throughout the spring, these shorter XC races are extremely punchy with dozens of accelerations per lap. My ultimate race fitness goals are later in the season, so these serve as a good way to build form.
I totally understand the point of structured intervals, it's all about trying to maximize the time you can spend at above threshold levels. I could probably make some more gains by doing them, but like Horner said, I just don't really enjoy doing them much. Most of my despised trainer time is spent just spinning out zone 2 rides, something that can be difficult to do consistently on my local roads.