Are you ever bothered about the format the DJ is playing the music on?

I notice many posters with "100% Vinyl" on them, and think to myself: why!?

As a DJ, it must surely be a huge pain the anus to lug a huge load of vinyls around as opposed to a little tiny laptop and maybe a back-up disc in case it fucks up?

And can people even tell when they're dancing to vinyl/mp3?