Recommendation Letters, to request or not?

Every year, a number of students show up requesting recommendation letters from me. There a range of students making these requests, on one side of the spectrum there are students which have only had a course with me, and on the other side, there are students which in addition to having classes with me, have had valuable research done under my supervision. Here are the criteria under which I will write a recom for you:

-If you are an undergraduate student, then you should have either:
  1. Worked on a research project at my lab or completed your BS project under my supervision, with a valuable outcome (I will decide on what is valuable and what is not) OR
  2. Passed at least two courses with me (in which you had done well) and have helped me out as a TA for at least two semesters.
-If you are a graduate student, then you should have conducted valuable research under my supervision (I will decide on what is valuable and what is not!). Recommending someone for a PhD or PostDoc position without mentioning anything about the applicants research capability is really useless.

Just to remind you, a recommendation letter is not positive and supportive of the applicant by default. It could include both positive and negative comments. So if you persist in getting a recom, then we may end up with a case like below*:

Memo to: the Director
Subject: Letter of Recommendation

1> Bob Smith, an assistant programmer, can always be found
2> hard at work in his cubicle. Bob works independently, without
3> wasting company time talking to colleagues. Bob never
4> thinks twice about assisting fellow employees, and he always
5> finishes given assignments on time. Often Bob takes extended
6> measures to complete his work, sometimes skipping
7> coffee breaks. Bob is a dedicated individual who has absolutely no
8> vanity in spite of his high accomplishments and profound
9> knowledge in his field. I firmly believe that Bob can be
10> classed as a high-caliber employee, the type which cannot be
11> dispensed with. Consequently, I duly recommend that Bob be
12> promoted to executive management, and a proposal will be
13> sent away as soon as possible.
Project Leader

(Later that afternoon another memo was sent…)

Memo to: the Director
Subject: Letter of Recommendation

Sorry about that earlier memo, Bob was reading over my shoulder as I wrote it. Kindly read every other line (i.e. 1, 3, 5, 7…) for my true assessment of him.
Project Leader

* I am not sure what is the proper reference for the above letter, I have seen it in different places. If you know the proper reference please let me know.