The Optimist;
There is another very good reason for PPLs not to fly paying passengers, their own safety.
Many of us on here hold CPLs/ATPLs and have thousands of hours and years of training and experience. I fly fixed wing and rotary for my living, recently in the rotary world there have been occasions when helicopter pilots have discovered that the paying customers of one pilot have been flown by a PPL who thinks he has been terrribly clever. The reports to the CAA have had little effect, a strong punch in the mouth and half a dozen very angry helo pilots pointing out the error of his ways seems more effective.
To any PPL who wants to share costs, thats great go ahead and do it, I cost share on lots of private flights with mates in our own aircraft. Cross the line and put passengers at risk and expect your world to become a very uncomfortable place if you are caught. For one thing your pax aren't insured because your flight is illegal, as one poster correctly stated you may be encouraged to fly beyond your limits, and the PPL is a very basic licence.
Cost share on an agreed basis and enjoy the flight. Charge and the pressure comes on heavily, the passengers expect you to perform when you may not be able to.
The CAA issue a very good leaflet on whether a flight is legal. It is aimed at passengers but is well worth a read by pilots too.