Robert and Flyk have it right, together. Robert is quite correct in his concerns surrounding spam, guerrilla marketing and the like. However, our worst possibility is also our best when it comes to that, there will be times when people paid to market software outright nail a question with a good, comprehensive and quality answer. Moderating this site is going to be interesting.
The stuff we want to get rid of is typically spotted by noticing a lack of engagement with the question itself, a lack of adherence to our quality guidelines, or both. Acceptable answers on this site will address a bullet list of requirements and constraints defined in the question. Great answers will meet or exceed every single one of them. But what do with the possibility of nothing meeting the question author's expectations? Should we simply leave those questions unanswered?
I tend to think no. If you know of something that meets almost every requirement and clearly state that what you're recommending does everything but something, and sufficiently engage the question, it might just be worth the mention. Now, this isn't to say "Hey my thing kind of does that so I'll put it here" - any time you don't meet the constraints of the question, you need to start your answer with "I don't think such a thing exists yet, but this does most of the job."
The example answer, while obviously written with good intentions has some problems. It doesn't address at least two of the must-have points, and it's clear that the author of the answer wasn't clear on some terminology being used. It appears that some clarification should have taken place prior to the answer being posted. I'm not sure that it comes close enough.
We have to come to terms with the fact that sometimes there will be no feature-complete solution. This is particularly important when it comes to free and open source software, where someone conceivably could write in a missing feature. Note, I said a missing feature, not 'roll your own Excel'.
I found this answer to my question about monitoring my Internet connection to be extremely helpful, in fact, I'm using the solution as prescribed right now. The thought of seeing such a valuable contribution deleted just because it won't run native on Windows 7 makes me very nervous.