tl;dr: yes, sometimes long answers are useful and might be necessary.
Case in point: my answer on the question about effective compression.
If you reread the original question, you can see that there are several things going on there:
- The OP has the basic question of why does this motor require higher octane but not this one?
- After a little reading, it's clear that there's a reasonably complicated approximation going on, clouding the issue.
- After a little more reading, you realize that the OP is using a poor approximation, leading to flawed conclusions.
- The OP seems to be in an information gathering mode, seeking further background on a complex subject.
Given all that, it would be super rude for me to just say "your effective compression approximation is wrong" and move on.
So, what I did was to write up a bunch of background, arrive at some general guidelines and then compare the original approximation with one that I feel is better. The OP and the community seem to agree that I did a pretty good job (which I appreciate) and I think I've helped the OP as well as others who are interested in the same subject. Remember: a good post is a better Google target.
Moving away from this specific example, remember that we are talking about cars now. No two cars are identical. Some cars that seem like they should be on paper can have wildly different issues (e.g., suspension geometry questions). We often have to give longer answers just to cover the bases. The fact that this also provides reference material for people writing future answers is that much more useful.
Obviously, a short urgent question like "help, help I'm on fire, what do I do?!" isn't the best candidate for a Tome of All Knowledge. For those, shorter is obviously better.