That's very true, but the amount of people who do insist on going that 10mph less than the speed limit on roads where it is perfectly safe to do said speed limit, it can get really annoying for the people stuck behind.Its a limit, not a target, one should not be forced to go the limit if they are not comfortable with it, ie 60mph on small country roads.
There are a few country roads that are 60mph that I have to go along when I visit my friends, no chance I'm going 60mph along there cos its quite bendy and it just wouldn't be safe, but I'm mostly talking about roads where its perfectly safe enough to go at the speed limit.