Personally, I think these questions should be allowed and for a number of reasons I want them to be allowed.
Why should they be allowed – the objective part
Questions like this are in line with the spirit of the site: this site is for the users of Mathematica and for the problems they encounter while using Mathematica.
Package installation problems are directly related to Mathematica (excepting those cases when the problem is obviously with some external tool that the package relies on and the problem can be made evident without even having to start Mathematica).
Whether a package is by WRI or others shouldn't matter, for as long as it is a Mathematica package.
Why do I personally want them to be allowed – the subjective part
I do realize that the OP would likely get a solution faster if s/he contacted the author of the package. I also do realize that similar questions often linger without an answer for a long time. But those are not good reasons to disallow them.
What I am hoping for instead is that:
- This site will become more popular with advanced users and package writers will check it more often
- This website will be instrumental in encouraging more people to develop Mathematica packages
- With time, these sorts of questions will get answered more quickly here
I do have a personal stake in this too: we get lots of emails about (installation) problems people encounter with MATLink (and I also get some about MaTeX). In the vast majority of cases these problems are due to errors made by the users, not problems with the packages. Dealing with these support requests takes quite a bit of time (we even considered setting up a community forum for MATLink to reduce the support load). I am sure that many would be quickly answered here on Mathematica.SE (even though most are not posted here). So I want these sorts of questions to be at least allowed.
Mathematica.SE is the most prominent Mathematica community. If the community decides that such questions are off-topic, that will have an negative impact on the package ecosystem. If they are allowed, then more third-party packages will be mentioned and discussed here; people will learn about and start to use more third party packages; advanced users who write packages and are not already participating will be more likely to join the site (and there are more such people than you might imagine); in the end more of these questions will get answered here. I think either decision could start a feedback loop and I want that feedback loop to have overall positive effects.