The whole thing sounds like a legal case waiting to happen to me. It may be legal but that doesn't mean a passenger won't try and sue both the pilot and the site should there be an accident.
On the face of it an experienced pilot taking someone along who wants to go to the same destination that the pilot is going anyway sounds like a good idea. My concern is that it is the low hour inexperienced pilots who will see this as an opportunity to save costs and they will be the ones doing most of the trips. This is likely to become a favoured method during the hour building phase.
I would ask people doing this what they think the point of the hour building phase is? This is where you go from new PPL to aircraft commander. You will make mistakes and learn from them, you will need to make go/not go decisions and gain the confidence in doing so. Having a paying passenger with time pressures to get to the destination is not a good idea.
Then there is what happens after that initial flight. If somebody wants to start it as a regular thing and offers an hour builder cash to fly him around will they say no?
Yes, it is the same as taking friends for a spin but the chances are they know your experience level and are more unlikely to sue.