Yea, it is certainly nice to know but it is also very dependend on conditions. What is it worth to know your best lap was a 2:17:8xx in Spa, when it was in cold weather at the end of a long race (fully rubbered track) with fresh tires and little fuel load? Then you have a number but it is (IMO) of no practical use and it's again up to you to remember yourself, that lap times in th 2:18s are already pretty OK, when you are driving in a more common setting.
Nevertheless, in hotlap mode it would be usefull as long nothing in the game changes that has an influence on lap times.