Back in the 3rd - 4th edition days, I used to go to the GW store in the Buckland Hills mall in Manchester CT to get in my 40K fix. There uses to be this guy who played a starcannon-spam Eldar army who would go trolling for newbie kids with their Space Marine armies so he could club baby seals, so to speak.
The point of this little anecdote is to illustrate that if there's an exploit to be had in the rules, there are people who will take advantage of it. Tournaments, at least, are an environment where the expectation is clear. They also shed light on these exploits, and GW has lately become responsive enough to do something about them in a timely fashion. Without tournaments, the exploits would just remain, and jackasses like the guy
above would be clubbing seals at LGSes across the country with little hope the exploits would get fixed.
Well, i think thats at least on part the fault of the store owner, letting this guy being bad for his business and the local community for not telling him to f*** off.
Looking only for tournements for balancing is imho a unhealthy approach. They are playing a different game anyway.