That is not true. Gavin Andressen supports alternate implementations. Take the following quote [1]:
"Diversity is a good thing. Diverse, inter-operating implementations of the Bitcoin protocol make the network more robust against software bugs, denial-of-service attacks, and vulnerabilities. There are several projects re-implementing Bitcoin; if you are a Java or Go or Python or C programmer who wants to see Bitcoin succeed, you should consider helping them out by reviewing, testing, or contributing patches."
He then goes on to list several alternatives, including btcd.
Consensus requires agreement, malicious attacks are orthogonal, unless you're hypothesizing a malicious party introducing an intentionally incompatible node and a significant fraction of users switching to it.
On the flip side one could argue that diversification of implementations is key to the long-term success of Bitcoin. If, let's say, there is a serious issue with bitcoind that shows up somewhere down the road that causes a fork and it is the only implementation everyone is using, there will be a network-wide problem. If there are a number of implementations in the wild, the fork would be contained to a smaller subset of users, and managed accordingly.
Getting to this point of diversification safely is difficult, however, which is why I imagine some of the core devs might be against it. I remember Amir Taaki got a lot of flack when he came out with libbitcoin.
if i run one without the bug and accept a payment, you and millions with the bug will just say "meh. this one does not match. will just drop it" and my transaction goes out of the main block.