So for me, as a guy with a 315w FTP, training at ~280w in 2 X 20's, 4 X 15's etc is wasted/junk miles?
That seems to be the spot I've identified as a very solid pace in a lot of RR's, circuit races and crits I've raced. I feel MUCH more comfortable at that pace as a result of training that zone and so far, at least in very fast group rides with 1/2's, I hang much better then when I'd be shot out the back quickly.
I wouldn't say its wasted, but why not perform work truly at threshold. That's 88% and technically you should be able to hold it quite long without much trouble. Most 1/2 crits are around a .9IF. I've seen lots of road race files with a .9IF for well over an hour, like you said. How is doing 20min intervals then even close to race specific when the power output is something you should be able to hold for the entire race? If someone can hold that for 90min, your intervals are only 1/4 as long as the max duration so quite obviously at a low power output for the given interval duration.
Sometimes intervals such as that are good sub threshold work early in the winter as part of a progression, but personally if I were trying to improve my threshold I'd bump up the wattage on those or do them for a longer duration. I've definitely done them on a trainer, but usually with some other component. 4x15 at that percentage would result in you maybe hitting 3-3.5mmol of lactate 10-12min in, so you're spending really little time at a high lactate level. Its quite different when you do them at 100% of FTP and with shorter intervals, operating at 100%-105% is a lot more effective. If I were doing intervals at 85%-90% I'd make them at least 30min long and often include surges or some kind of stochastic component.
I did spend a season doing a lot of SST work in that range, but found that 2x30s and 3x20s were basically the only sessions that seemed to have a high enough duration to really translate into anything substantial. Maybe you're different, who knows. I also don't know your weight or what kind of courses you ride on, so perhaps those are useful variables in the answer to your sort of question. Other important variables would be your training volume, frequency of intervals, training history, CP/MMP curve, etc. If you're riding 20hrs a week and doing those during 3 sessions then maybe that makes sense for this time of year. Or maybe your CP5 is very close to your CP20 (but then why would you not try to raise your CP5 instead) and the sessions might be harder physiologically than they are for most people.
Don't take me too seriously. GramzStrava