A lot of the older generation of commentators started simply because they were the only people with enough knowledge to do the job when cycling made TV time in the past, and/or had contacts which helped them get the job. This genaration are now showing their afe a bit. For me David Harman is one of the best, I don't watch ITV4 coverage much so can't comment on L&S properly, though the little bits I have heard sound a bit dull, althogh PS does understand what is going on much like Sean Kelly on ES (but not quite so good). Not convinced by Mr McCrossan, find him a bit monotone, but Brian Smith is excellent at identifying riders, which is helpful to new viewers (and old ones as well!!).
It is a difficult job - imagine having a five hour stage where not much happens until the last hour or less - but even so a professional commentator is employed to keep the chat going and do their best with dull stimes. There are plenty of those in a long race. Constructive comment helps, just moaning about a few errors will achieve little, always thinking you don't know how good something was until it's not there!