Interesting topic to bring up, as on the sim forums we are discussing are they real enough and are they getting any realer?
Basic Flight Simulator package will teach you the basics of flight but nothing about electrics and engines etc.
However, such packages as PIC767 (which is excellent) DF743 PIC and PSSA320, if one learns these correctly they I think they would have a great deal of knowledge which could be put into practice in the real world.
Many of the serious simmers I know, have taught me the systems of the 767 almost inside out, but me being me still don't catch on to them. As ****** Thing said about a certain addon and if learnt correctly, it would be fairly easy for anyone of these guys to take over.
Flight Simulation sites are becoming more and more common. There is a wealth of knowledge online that can teach one with no knowledge to someone who could get their ATPL CPL etc easily.
As for the organisation with over 50k members that serves as virtual air traffic control and is great fun for the simmers. No where else can I be on London CW Centre climbing out towards Alicante with a few others while everything goes on around me

lol
I can vouche for most guys and say they are just people who are interested in aviation and have past their best before date for becoming a commerical pilot. But I do admit, anyone can get this knowledge and use it wrongly!!!
But I suppose it is one of those things we will have to live with like hankies and pens onboard aircraft. Anyone can write "bomb on board" and leave it in aisle to cause havock like the UA767 a couple of weeks ago.