now there's a term from cryptography.
I assume that because you've used the term, you're familiar with it, and hopefully we can discuss from there. Security through obscurity does not work in cryptography because historically those with enough resources and determination can always reverse engineer the process eventually, either by capturing an encryption machine to understand it, or by working it out methodically and slowly using a number of very mathematical methods. A key example would be the Enigma machine (ignoring the numerous iterations and versions of it). Relying solely on obfuscation with encryption is very bad practice because once they find out, all of your communications past and present are an open book.
This is why encryption relies on another very important bit of information, a key. The enigma had this too, in the form of codebooks at one point in time.
Of course, this doesn't work for a computer game. You can't really store a codebook on the computer without it being available to a hacker [especially after the code is presented that might show them exactly how you've been storing it], and if you don't have a secure channel to send new codes by (which you dont if steam is compromised too by the same argument I'm about to put forward) then you're up the creek.
So you get to the quandary of trying to use encryption without having a second secure channel, which is mirrored in the case of Enigma by the germans when at one point they sent the key to the enigma codes twice preceding the message (very very very very bad).
So in terms of encryption, from my admittedly not expert knowledge, this is a very bad thing since there's absolutely no way around the way that in this case, the user holds all the keys, and without the obscurity, there's no way of stopping them from copying the client responses exactly. [and that goes for the responses of hacked clients pretending to be ok clients in the eyes of steam]
Of course, the real issue is probably more likely to be things like the simple possibility that someone will uncover an issue like a buffer in the netcode that can be overrun, and start using HL2 to execute illegal code on other clients. Perhaps they'll use that vulnerability to change steam so that it accesses their own steam master server (which they now have some of the information for how to write, if not the exact code) and can start quietly downloading other trojan software onto your machine using that lovely intelligent (and automatic) download system. Of course, the modified steam client might not show you visibly everything that it's downloading, might not actually allow you to turn it off, and you might now be rather paranoid about the possibilities of the whole thing.