Quite a flawed and naive view in my eyes.
First of all, the developers are not locked away in a vault somewhere with no access to the outside world. The genuine release date, which publishers will need to release to distributors, retailers, marketing departments and so forth, will become known to them.
Second of all, not all problems requiring patches are simply a case of a developer being lazy and not getting a game ready in time. More development time doesn't necessarily mean all bugs will be eliminated; in many cases, some issues will only ever get discovered once a game is released and the public is let loose on it.
Furthermore, even if such an undertaking were feasible it could potentially be counter productive if it meant that developers rushed to hit this 'phantom' deadline, perhaps spending too little time on the design phase because they were desperate to get started on coding. Thus even if they suddenly got another 2 months to finish things it could be too late by then, if they have built everything on top of a rushed and ill thought out design.
As mentioned by far the best option is to wait a few months (or more) before buying a game, that way it will invariably have been patched, usually be cheaper, and may even mean you have faster hardware to run it on.
Or elevator glitch in MW2.... takes some serious down time.