I think apart from personal preferences, the way how a question or an answer is altered and how links are embedded is crucial in any edit.
The main reason for carefully considering the extent of an edit is given in the comment section of the question: Your edit may change the answers that are given significantly, to the point of being useless to the one who formulated the original question! And the main goal should be to get a good answer to whatever the OP was asking, on a level appropriate for them, and not about what someone else deems interesting or understands.
I will give my personal view (I am not an authority above and beyond the community, I am a community member with more-than-normal amounts of tools to do what every community member should do):
There are a few things to say about links. This is what our help center has to say about them:
Provide context for links
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the external resource is unreachable or goes permanently offline. Links to other websites should always be helpful, but avoid making it necessary to click on them as much as possible. (source)
This means a link should not serve as explanatory device on its own. If one wants to include explanatory content or a clarification/definition of terms, the proper way to do so is including the relevant information in the body of the post and using the link as a reference for that content.
Whether or not Wikipedia is a reputable source is up to debate but pretty much depends on the article and assertion we look at. A main advantage of only taking specific relevant content and referencing it is that one can see whether this specific content is backed up by references. On the other hand, the IEP and certainly the SEP are always properly sourced and referenced.
I think that while it is ok to alter a post in principle, even substantially, and is totally within the rules and to some extent encouraged, it very much hinges on how it is done. I'd like to draw some lines I see as crucial.
- Do not put words into the mouth of a poster. While it is ok to add examples and make the whole thing more accessible and comprehensive, one should not make it look as if this was the line of thought of the original poster. It isn't. Added content should always be in objective language.
- Do add to and expand on what is there, not change with what you think might be there in mind, see also next point.
- Ask for clarification in comments first. You should be pretty certain of the intentions and needs of the original poster if you are to make substantial changes to the content. It is their post, after all. And many users do not know the mechanics of the site well enough to be able to roll back (or make) an edit.
- Keep in mind that it should be expert knowledge reflected in answers. Not everybody has to understand every question and most people will not bother to even try to. Those who want to can certainly hit the first search engine hit for the term (almost certainly Wikipedia) themselves. Thus, technical terms, given they are not ambiguous, do not always have to be explained in order to make a question a (very) good one that solicits great answers. If a definition or qualification is warranted in the context of the question, more specific sources that might not be that easy to find would probably be more helpful.
As said, this is my personal view.
The main four takeaways should be:
- Links should never be a stand-alone but always be used as a reference for content of the post. The information that is necessary should always be part of the post proper, not hidden somewhere in links.
- Wikipedia links are, given point 1 is respected, of limited use not necessarily because they are wrong or unsourced (both things happen with regards to philosophy quite often) but because they are literally the information that is the most trivial to find and because they give a lot of information that is completely irrelevant for the post at hand.
- The main scope of any edit should be the interest and needs of the original poster already manifest in post and comments. Everything that goes above and beyond that should be checked with them first.
- Never pretend to speak for the poster.