There is a FAQ on the main Meta: "How do I write a good title?" which has a number of excellent answers (with 100's of upvotes, and fewer than 10% downvotes), the only one with a negative vote total specifically mentions "question mark"; the other answers imply its usefulness. Sometimes you need one, rarely you don't; if a question mark alone is going to make or break it then you probably want to improve your title.
It's worth reading that FAQ but one short answer outlines some useful points:
Your title should be, in order of greatest to least importance:
- Searchable. The point of Stack Exchange is not only to help the asker, but to others who may have the same problem as well in the future.
- Descriptive. A searcher would like to know whether the question is another one of those "How to write a HTML regex parser?" questions or actually the "Why is parsing HTML with regexes a bad idea?" they are looking for.
- Short. Put your 10-page essay in the post, not the title.
- Precise. Tell us in
- as few words as possible
- as much as possible.
- Interesting. How else will you make the Hot Questions list?
Remember that raw links included in a Q&A (but not in a comment) get automatically converted to the title, so it's useful to have a good title that is meaningful on its own.
For example, this gets converted:
Replace trilogy raw links with the current question title?
That's a statement ending in a question mark, it could be a question (asking if others agree) but it's a request.
Long ago this was asked: Should the lack of a question mark affect the question's quality score? - they made a Data Explorer query to compare Score and Favoriting with the presence of a question mark:
Count, Sum, FavoriteCount, AverageScore, AverageFavoriteCount,"?"
1017086, 932969, 328823 , 0.91729607919, 0.32329911138 , 0
3826947, 6565454, 2220523 , 1.71558529553, 0.58023353864 , 1