When I started flying, I was trained to use a flightplan from the very first navigation flight. Not all students did this, however, to me it felt safe having a bell that would ring if "my time was up". I know plenty of pilots who aren't that keen on flightplans, but I don't understand why. IFR flightplans are a different thing, they are "actively used", VFR flightplans are "passive" documents, they are there "in case of".
It's a very good point by SSC: the purpose of a flightplan is to be able to track you down in case of doubt. 99% of the time everything's allright, a detailed flightplan seems unnecessary. But what if...
I consider a flightplan a document stating what you're going to do on this flight. You leave it behind and from the moment you open it, it's a ticking clock. You close it when reaching destination and that's it. But if you don't, it will ring a bell to some people. And THEN your flightplan becomes important, since it is THE source of information for the rescue services. Who is this guy? What was he doing? Where was he flying? Where should we go looking for him? It's a document where you, as PIC, leave all information required if someone would have to come looking for you. I consider that very important... cause I'd like them to find me as soon as possible
. And yes, a mobile phone number is very usefull information, since if something happened... you'll probably be on the ground too.