Although crits and cross races are both hard and short, they really aren't that much alike. There is no recovery in cross. Seriously. In crits, you can sit in the pack and really not work much. And then you accelerate out of corners and for primes. The other difference in crits is that your placement in the pack wil determine how much you can recover. Sitting second or third wheel, you can recover most of the race if you want, because you can corner at speed, eliminating the need to accelerate out of the corners. If you're at the front of the pack, you have a constant hard effort (no surges). If you're at the back of the pack you're working hard, hard, hard with lots of surges. My PT files for crits are a series of 30s on/30s off for most of the race, with power during the off being close to zero and power during the on being 400+ watts, however my HR is well below red-line for most of the race.

Cross, otoh, is all on, all the time, with the exception of courses that really limit your speed (like the chicanes through the trees we had last week). Very, very different physical demands, although I can see why if you've never raced either and just spectated you might think they're similar.

So, for cross, you need the 2 x 20s so you can train your body to sustain a hard effort and shorter intervals to respond to the push on transitions. One of the best places to gain (or lose) time on the field is out of the transitions (corners, post-barriers, flats after a descent, etc), so doing 15 second intervals is extrememly useful, but you don't train with full recovery. Rather, it's 15 seconds on, 45 seconds off, repeat. A typical PT file for cross races would be red-line HR the entire race and pretty high power as well, with short bursts of even higher power.

Road races are even different, because you typically have a variety of terrain (hills, rollers, flats) and the pace and intensity will be dictated by this, as well as by the effort of specific racers (attacks, etc). Positioning also plays an important role in the amount of effort you need to produce. Of anything, I'd say the demands of road racing are much more like the demands of mtn bike racing. The primary difference (at least in women's racing) is that you don't typically do a lot of drafting in mtn bike races, so there's not as much recovery as there is in road races. But both are highly aerobic efforts with short intense periods based on terrain and tactics.