In a single engine aircraft - which with a PPL I assume is the case - what you've said is true. Runway behind you is worthless...
In two, three and four engine aircraft it's a different story, particularly if that aircraft is certified under FAR / JAR 25.
Intersection departures are perfectly safe - and many times quite cost effective - provided that you have calculated the take-off and climb performance and are able to meet any limitations, etc.
Before I get beat to death about this explanation by the masses, the above is a very simple and brief explanation directed at a PPL who is not attempting to operate a transport category aircraft. There is, in fact, a lot more that goes into it.