Sorry Roy, I will disagree with that, depending on your idea of "much past" LTHR. I would consider much, 10 BPM and my LTHR is around 156 and max is around 184. The 20 second sprint thing is why power meters are such a big deal but you're HR will still continue to rise after that 20 second effort. 2 minute max efforts is when I start noticing higher heart rates... or any time during a cx race.
Anaerobic efforts are not maximal efforts, which is what you have described. Anaerobic efforts are efforts that aren't sustainable by aerobic metabolism and dig into energy stores, like glycogen. Basically, anything under a 1-hour max effort is considered anaerobic, since the threshold number is what you should ideally be able to for a 1-hour max effort at a steady effort and the threshold effort is the divider between anaerobic and aerobic efforts.
Also, training past threshold is a big deal for racing, it's even important for longer events like ironman. Most interval training is aimed at surpassing threshold. If you never left the aerobic training zones, sub-threshold, you would never train your anaerobic system so any efforts under 1-hour would be less effective than if you trained them. A typical XC ride is generally a large number of anaerobic efforts in succession.
Slimm, wrong. being off by only 5 BPM can put you in the wrong training zone so it is a big deal to be more accurate for maximum efficiency of training. It become more important around Lactate threshold because there's not much room between hard aerobic efforts and anaerobic efforts. For instance, I can hold 155 BPM for around 1 hour. I can hold 160 for maybe 30 minutes if I'm well rested, like when doing threshold testing. The calculated HR zone is too inaccurate, enough so that you could be a full zone off pretty easily. I assume that since he was willing to pay for a HR monitor he's willing to figure out the actual zone, it's easy to do and it can be your workout for the day.
If you only ride once or twice a week, calculated zones are probably fine because you probably aren't riding for training. I would also say that at that point a HR monitor probably was a waste of money and you would be well served by just using RPE (rated of perceived exertion) and a watch.