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Excellent points. I have one comment to add.
More likely, you'll have a conversation like this with your internal customer (be that your management or product management). In dysfuncitonal organizations (i.e., in most companies) standing up to them can be very scary.
I agree, scary and politically dangerous, although not doing it can be just as dangerous. I want to share a point of view though, which took me many years (and many 80 hour weeks) to realize. In short, you probably have much more power in these situations than you realize.
I have been in many a dysfunctional organization, and 9 times out of 10 by the time the project/product manager gets too you to ask for the new feature, they have already promised it (usually because they were pressured by those above them who have promised it already as well). However here is where you need to realize that while you are the bottom of the food chain, things are not as bleak as they seems. Because what that really means is that in order for your manager to look good, and their manager to look good, etc etc on up the chain, you have to come through. In other words, you have them by the you-know-whats. Now I do not in anyway way advocate abusing this fact, however I do think it can be used (much as they are trying to use your overtime to make themselves look good). This is the time to sit up and tell your superior exactly what dws is advocating, basically "If you want this to work, you need to re-proritize". You need only stand up to your manager and in a calm clear manner explain why this will ultimately come back to haunt them (it is critical to make it about them here, not about you). It is then his/her problem to run it up the chain, but if you give them a lot of good information, reasons, examples cases, what they are asking is a bad idea, they will have plenty of ammunition.
The critical thing here is to not sound like a nay-sayer, do not be bitter, and be sure to site examples of where things went wrong and where things went right in the past. Avoid blame (always a bad idea) and try to be as cold and logical as possible. In the end if your argument is solid, you may make some headway. And if you don't you have said your peace and stood up for yourself, and hopefully things will be better the next time.
And worse case scenario, you get fired, and you go get a job at a porn site ;)