You don't understand why the consensus critical code that EVERY single node in the network needs to run cannot be modified at will by all sorts of different implementations!?!?!
I don't know what to tell you.... think harder? You do understand the implications if certain nodes do not agree on the rules ?
What you are saying is true, obviously.
The advantage of several implementations is that rule changes are done more 'democratically'. Contrast this to the current debate where the 'core' doesn't want to raise the blocksize limit right now but others do, and those that do are forced to implement code from Mike Hearn, who not everyone trusts.
We don't need democracy! Please stop trying to stick its failures to Bitcoin as well.
We need expert consensus on secure and properly vetted code. Having more idiots "vote" on what should go into the consensus code doesn't help with the process. I know that's what Mike Hearn will have you believe but that's just plain wrong.
Not everyone's opinion should be weighted equally in this decision and it's a dangerous idea to try an introduce more "voters" "because decentralization".