Well, I would have no problem saying, “Please do not hesitate to contradict Leaffan”! 
To be a bit more serious, I think that several things are being confuted in this thread.
First, there is the issue that some constructions are descriptively ungrammatical, that is, while intelligible, they are ways that would never be said by a native speaker or even a fluent ESL speaker. “Me go to the store” marks the speaker as a small child; “Is disliking this dish by you/”, a person of East Slavic extraction and halting use of English, trying unsuccessfully to literally translate a particular Slavic idiomatic construction.
Second is the issue of varying registers. A usage seen as normal and acceptable in one context may be as out of place as breaking into the chicken dance in the middle of a stately ceremonial ritual in another context. Colloquial speech in various dialects including Standard American and Home Counties English as well as regionalisms, formal written English in varying standards, etc., may well have varying grammars. “Me ‘n’ Joey’re gonna go bike ridin’., ma, okay?” is not an erroneous construction – it’s good colloquial usage. But it is incorrect on several grounds in a more formal register – and that’s not prescriptivist pedantry; it’s describing what is sociologically expected by native speakers using the two registers.
To give an example of a solecistic construction not referencing the colloquial/formal dichotomy, contemplate “I then crossbred the two strains for three daughter generations, and came out with the following array of characteristics.” This would not be out of place in an oral presentation concerning an experiment or an article on that experiment in a magazine, or in a letter about the experiment. But it would be very much incorrect as a part of a monograph or report in a scientific journal, where the expected “register” is to eschew the personal and report the procedure and results in a manner that ignores who conducted and reported them. “The two strains were then crossbred for three daughter genrations, with characteristics resulting as reported in Table 2 (q.v.)” would be the preferable statement in that register – not that the first-person statement is ‘ungrammatical’ but that it’s inappropriate for the ‘register’ of the chosen medium, the scientific journal.
The “rules” of prescriptivism serve a valid purpose – to define for those not accustomed to using a given register, say formal written English of business letters and magazine articles, which forms from the colloquial register with which the prospective writer is familiar are appropriate and inappropriate in the unfamiliar register. In some ways, they act like etiquette references: : a person wishing to avoid giving offense and complying with social standards has a means of finding out what those social standards are. There’s nothing “wrong” in some abstract sense about violating them, of, e.g., addressing the Archbishop of Canterbury formally with “Dear Rowan”, but it may be counterproductive to the desired result.