It really depends on if you’re talking about a Daniels tempo, supposedly run right at LT2, or a tempo the way most coaches have historically used the term—a continuous run at any of a range of moderately hard paces.
The Daniels tempo fell out of favor a long time ago and was never universally adopted. There are a number of problems with it. One is that it’s just really, really hard. 20-40 minutes at LT feels pretty close to a 100% effort if you don’t have the context of a race to hype you up. In fact, a common test for determining your LT2 heart rate is to just run a 30-minute time trial, solo, as hard as you can. Your average heart rate for the last 20 minutes is supposed to be your LT2.
Another problem with the Daniels tempo is that nobody knows what their LT2 pace is. The tables are not reliable because there is very wide variation in % of VO2max where people hit LT2, and that’s especially true for younger runners. 1-hour race pace is also not reliable because even collegiate runners typically aren’t strong enough to hold LT2 for an hour. Even if you did a step test in a lab, you’re probably getting at best an estimate that’s within 10 seconds of your true LT2. (For a lot of reasons I won’t go into right now, lab tests for LT have very poor accuracy.) Then consider that LT2 varies day-to-day. This is why the biggest innovation in the Norwegian method (though it’s not that new, and it’s not even unique to running) is the regular use of lactate testing in workouts to monitor intensity. They know you can’t just plug in a pace and expect it to be reliable every day for a month.
Broken tempos make it less important to hit LT2 exactly. You can err on the side of going slightly too fast because the lactate clears quickly in your short rest intervals. That way everyone ends up with a slightly oscillating lactate curve that should average out pretty close to LT2, but nobody is getting steadily accumulating oxygen debt, as happens anytime someone does a straight tempo even 5 seconds per mile too fast. And, yes, you can get more total volume in.
As for slower tempos in the neighborhood of aerobic threshold, these are, if anything getting more popular. These are the slowest paces at which you elevate blood lactate above baseline and reduce muscle oxygen content. I won’t go into too much detail here, but it is believed that both changes are signals for peripheral aerobic adaptations that don’t happen in response to merely easy running. In other words, it’s the easiest “quality” running you can do.
Another great benefit of these slower tempos is that they allow you to get more workouts in per week. Many, perhaps most athletes, cannot really handle 3 hard workouts per week. Making one of these workouts an aerobic threshold tempo gives you that extra stimulus without going too deep.
Aerobic thresholds are not so hyper-sensitive to exact pacing. You have a pretty wide range of paces where you’re in the general zone, and going 10 seconds/mile too fast isn’t going to suddenly turn the workout into a monster. This makes them appropriate for large programs where it’s hard to customize workouts for everyone, it allows groups to run together, and it gives athletes flexibility to vary the pace depending on whether they’re feeling strong or weak on a given day.
Finally, aerobic thresholds can be done indefinitely. Daniels tempos need to be carefully periodized, but you can run at your aerobic threshold once or twice a week, just about every week of the year.
Of course, nobody knows what every other coach is doing, but in recent conversations I’ve had, a number of pro and college coaches have told me that they think an important recent development in training (at least in the US) is an increased emphasis on aerobic threshold rather than anaerobic threshold.