I play with a group of guys who are always complaining that people are cheating with hacks when playing Vs mode in  Left 4 Dead PC version. My question to you is  how much of this thats actually going on? Do you ever cheat in a game? Why would you cheat in a game? How secure are the L4D servers?

I've ran into a hacker twice the entire time I've played L4D and thats it, and the one I know got their steam acc banned. There's barely any on that you'll run into.

I'm glad to see not alot of people have not been having a problem with this. 

I was playing some vs today, our team was getting whooped on and of my teammates messaged me that one of the opposite team's players were getting illegal shots though the walls, doubt it tho.

I've had problems with hackers in PC games over the years sometimes.  Some of it was blatant stuff like aimbots and wallhacks, other was just glitch stuff like getting below the concrete in the omaha beach level of Battlefield 1942.  Console games get these 'exploit' hacks sometimes too.  Xbox 1 had real cheats as well because that system was majorly hacked.

I've seen screenshots of L4D with different cheats enabled, but I don't know if a newer version of VAC might have wiped it out or what.  Most every PC game has hacks available, but since VAC, Punkbuster and all other cheat protection is catching up, hacks have to advance too.