I think it depends on what intensity you are talking about and how long you are training at that intensity. Many ascribe to believing that 80-85% of MHR is to easy to benefit from and too hard to recover from but avg. HR's during a 2.5 hour race are usually in this range and typically, if you are watching your HR while in the pack, it is near this range. Granted it is and average for a whole race and there are fluctuations but I think that it can be very beneficial to train at this intensity during long tempo like efforts.