You'd have to tumble down the road and hit your head again in the same spot for that to be much detriment. Most fibre shells soften to their fabric state on the point of impact, so they too will offer little or no protection if you were to tumble and hit the same spot of the shell again. The benefit of fibre shells used to be their ability to stay together after the initial impact where plastic ones would come apart.....however, that isn't the case anymore as plastic ones can stay together too nowadays.
The EPS liner has a one time energy absorbing factor. Once it is crushed, it's junk. SO regaining shell shape doesn't mean anything if the foam underneath the shell has been compromised anyways. See the video and you will see an example of a helmet that has seen a huge impact, and shell looks undamaged, but the liner has been compromised inside.
GO to the 2:00 mark in this video to see what I mean- plastic shelled helmets do better in these tests than fibre helmets....10 yrs ago, a plastic helmet would crack or shatter.
[video=youtube;VA63gMKU6PM]http://www.youtube.com/watch?v=VA63gMKU6PM[/video]