As the question stands as of now, Revision 8, I believe it does fit our Help Center's on-topic guidelines - 3 out of 4 points:
the written and unwritten - but well-established and expected - rules or conventions of behavior in a specific setting (also called etiquette).
understanding social norms as they relate to interpersonal interactions - why do we interact the way we do?
understanding theories and concepts commonly associated with interpersonal skills. - What is [theory/concept] or How do I use [theory/concept] to achieve [goal]?
That the body is large, and does carry a 'rant' flavor doesn't help the situation. However, we also ask for more details and context repeatedly, so shouldn't complain when we get it. Again, this complies with the Help Center's guidelines:
Questions must be specific enough to be answered
Because interpersonal interactions are very complex, we require that questions be specific - and preferably that they relate to a situation you are actually in. Please include the following information where possible, and to the degree that you are comfortable…
The OP has clarified what, precisely, the question is:
My question therefore is this: is adding such an obvious sentence necessary?
and, to reiterate and re-focus:
The question is simply this: given that you know that somebody is definitely capable of doing what you ask them, and given that you specify clearly what you want, is adding such a line to an e-mail necessary?
It is, thus an etiquette question, at the minimum. The obvious choice for an answer is wither "Yes" or "No" and how well those answers are supported should affect their votes, not the question's worth.
Yes, this question does belong here on IPS, and would need revisions to be appropriate for the Workplace stack, as I understand it anyway.
I do believe that the question, like almost any other on any stack, can be improved. I don't believe that it needs to be improved to remain open, however.
In specific response a "burden of proof" statement I'll offer that:
- etiquette includes written communication as much as face-to-face, if not more in the current age (Netiquette comes to mind here)
- as per the 2nd bullet in the quoted answer, the OP specifically asks if the "rule applies in a given (extremely precise) situation"
- the OP, and many answers and comments, have affirmed that, applicable or not, it is a "convention of behavior" when writing such emails (even old-style hand-written and typed-on-paper memos have the same convention)
Seems to me that the "burden of proof" has been met, and implicitly accepted by the actions of several other users.
As said earlier, the question does carry a ranting taste, which is unfortunate. I don't believe it is a rant, however. Rather, I think the OP has taken the time to be very specific about the situation and has attempted to remove any chance for misunderstanding from their post. ("Better safe than sorry" seems to have been turned on its head here.)
Keep the question open on IPS.
(Final answer, Alex)