Imagine this scenario:
Bob is 24 year old private pilot with 85 hours. He got his ticket last year and works in IT. He's kind of bored with his job and hopes to one day fly for the airlines. It does pay for some flying, as he can afford a couple hours every month to fly. He knows that it's going to take more than a few hours every month to get him where he's trying to go, not to mention the cost of an instrument rating, commercial, etc.
Bob discovers the website part134andahalf.com. He can post his flights, and solicit the general public to join him on flights, sharing his expenses. "Wow", he thinks, "I'll be able to build flight time in no time!". His $160/hr cessna 172 rental could potentially only cost him $40/hr if he can find three passengers. So he signs up and posts his flights. He doesn't have anywhere in particular to fly to, so he posts a flight to a popular summer destination only accessable by boat or plane. He posts the trip for this Friday, but as of thursday night, no takers.
So he cancels his flight for friday and posts the flight for Saturday morning.
Sure enough, Friday afternoon he gets an email from the website. Two people have booked two seats. He emails Karoline, who booked a flight for her and her boyfriend Jon. He tells her to meet him at the local airport at 8:00am Saturday. She emails him back and agrees. Friday night she emails him and asks if her sister Lauren could join them on the trip, offering to pay him cash for the third seat since she didn't book it through the site. "Sure", he replies, figuring that weight will be tight, but he can ask the FBO not to top off the plane from it's previous flight. It will only be an hour out to the island.
Saturday morning Bob checks weather for the day. It's typical summer weather, hazy with a chance of T-storms in the afternoon. He decides he's OK with the weather. All four meet at the airport as planned. He realizes he forgot to ask for Lauren's weight, and determines that Karoline is likely a good fourty pounds heavier than the buck fourty she claimed on the trip request. On top of that, the flight before him in the 172 topped off the fuel, and the FBO claims they can't offload. He looks at his weight and balance and figures they'll be OK; he once read on the internet that the 172 flies perfectly fine 30% overgross. Let's fly!
The flight to the island goes as planned. Bob parts ways with his passengers, and they agree to meet at 6pm to return home. He goes off and does his thing. Around three o'clock he checks his ipad for weather, and notices a lot of convective activity forming to the west, moving eastbound. He figures if they can take off by 5, they can beat the worst of it. He texts his passengers and asks if its OK to leave at 5pm. They mention that they're on the other side of the island and it will take at least an hour and a half to get back to the airport by bus but will try to get back by then.
5 o'clock comes and goes and the trio has yet to arrive. Bob finally gets a text from Karoline letting him know they're 15 minutes away. He preflights the plane for a speedy departure. He checks the weather again; the storms are building. Finally the passengers arrive and he loads up their baggage. He looks at the radar, and notices the line of storms has now blockaded his route home. He makes an executive decision to let the storm pass before lifting off. As such, he ends up spending the better part of two and half hours with his passengers. Luckily the airport cafe was open so they all grabbed a bite to eat. Lauren wasn't happy about the delay, as she planned to meet her boyfriend for dinner.
Throughout the meal Bob checks the weather for improvement. The rain comes, and cells start to break up. By 8 o'clock he looks at the ceilings, visibilities for his route and it looks iffy, but figures he should be able to pull off the flight. He's not night current so he really want to take off right away. He loads up the gang and plan heads off for departure. There's a line of departing aircraft, and by the time he's wheels up its about 8:30. It's getting dark, and he finds himself having to dodge clouds on his way home. Finally the sun has set, he's over the water, getting bounced around and doing his best to avoid buildups with no horizon to speak of...
I don't really need to finish that story, but many of us here has scared ourselves at one point or another. We've also had to cancel flights, disappoint passengers, etc. It's not an easy thing to do, and the dynamics change when there's an exchange of money, even without a profit motive.
Should he have scrubbed the return flight? Taken off without the passengers? The 135 and 121 world have dealt with these issues for decades. As such, they're required to have more capable pilots, flying more capable aircraft, with a backbone of a dispatcher to help with the go-no go plan. They also have op specs that guide what they can cannot do. Air carriers have a hard enough time dealing with these issues; is it realistic for us to expect a 86 hour pilot to be able to do the same?
The point that I'm making is that the business model for flight sharing, as proposed, will result in more accidents, a lower public opinion of general aviation, and ultimately more regulation. Be careful what you wish for.