I am actually having trouble understanding the examples you give as evidence.
This comment, which seems to be one of your examples of the problem you are asking about, does not appear to me to be written in an assertive tone:
You can of course email the organisers and ask if you can participate without paying I would not actually recommend this unless you have a reason much more compelling than "I couldn't get funding". I think it will simply annoy the organizers, who are usually influential people whose bad side you don't want to be on.
Frankly, I don't see any assertiveness in its tone. It actually includes several qualifiers - "I would not recommend this", "I think", "usually".
Whereas to me, your response does seem to have an assertive tone. Instead of saying in a more reserved way "in the conferences I attend this is different from what you report, and here's how", or "this is not true as a general rule", you appear to deny the other person's experience by saying it is "certainly not true":
@NateEldredge, certainly this is not true. The organizers are usually not influential, but local academics which are not influential. The most influential people are usually the PC members and especially chairs. Not organizers. Emailing the organizers with a reasonable reason is a good advice.
Your main proposal seems to be that users should use a less assertive tone. Perhaps you could rewrite that first comment you gave as an example to indicate how you would write it in a less assertive way?
Finally, when a user asks about how something will be perceived by other academics, I think it is useful for them to know if the response will be emphatic, and if different people have passionate and diverging responses. To quote a comment by user2390246 on one of the other threads you find objectionable,
Yes, very nice answer. The range of (quite passionate!) opinion expressed on this page is a clear indication that your mileage may vary. So, OP needs to find out a) will such a letter help him/her in their particular situation, and b) will the professor be able to write them a strong letter? @BenWebster's suggestions for how to do this are spot on.
Note also the element of "gamble". There clearly (as evidenced here) exist people who would look very favourably on such a letter, and others who would consider it very negatively. So having canvassed opinion, OP needs to decide whether it is worth gambling, or whether they are better off going for a set of "standard" (but hopefully strong) recommendations.