Although your idiom request was quite clear, your meta question is of interest regarding ELU questions more generally.
This answer just addresses the issue of adding sample sentences, but the principle behind other requirements (e.g. show your research) is similar:
Good answers often take a lot more time and effort to produce than it took to ask the question.
It is therefore only fair that the crafting of questions is held to a high standard.
On this site (ELU), a guess at (or statement of) the requested word or idiom on its own is often considered insufficient as an answer to questions tagged single-word-requests, idiom-requests and the like. Dictionary entries, arguments for and against, Ngram links, or references to other accepted authorities are par for the course. This is also the case more generally - answers are expected to be backed up by research or logic.
After going to the trouble of finding a word or idiom to satisfy the request, it's a little disheartening when the OP (original poster) rejects it because that's not quite what I had in mind. Consider Mari-Lou's two examples in the comments to the question:
- Friend: You really _______, haven't you? How do you think you're going to pass the exam next week?
- Bob: You shouldn't have ______ money. Now you need to fix your car, where's the money going to come from?
The phrase ran your well dry (adapted from K1eran's answer to your idiom request) fits the first but is clumsy in the second. On the other hand, the past tense squandered of matt's single-word answer fits the second but not the first.
Adding a sample sentence means that those who answer can simply drop their answers into the blanks to check if they fit, and avoid the research if they don't. The answers tend therefore to be more satisfying to the OP, or else easily down-voted since the OP's intent is clear.
A clear question, an answer that clearly answers the question, community support for the answer by up-votes, and the green tick of approval all help the broader community and contribute toward the fundamental goal of all Stack Exchange sites: "We build libraries of high-quality questions and answers". When a visitor searches for the same question and lands on yours, all these factors contribute to helping them decide whether the answer is useful for them.
And it all starts with clarity in the question.