There does seem to be a bit of logic behind the idea of only breaking the rules you understand. Grammar rules are conventions--breaking those conventions is going to have a particular effect on the reader. You ought to be making an informed choice when you mess with readers' minds. Lu would appear to agree with that point--however, instead of simply naming problems or correcting them, she believes in walking the writer throughout the process, actually helping them to become informed.
Some "grammar errors" are innocuous. Comma splices and run-on sentences are sometimes more effective than their grammatically correct alternatives. When I'm grading papers, I ignore them. When I'm working in the writing center, I frequently point them out and ask the writer to consider whether strict punctuation is more important than the rhythm of the sentence. Most people assume that the correct punctuation is better, even when it reads worse, so they "fix" it. I don't blame them--some professors write assignment sheets that call contractions errors.
Lu insists that we ought to spend this time with students, rather than accepting the fact that we can't. But how useful would it really be to spend twenty minutes on every error in a three-page paper? For one thing, even if you approach it from the standpoint of style, grammar isn't everything. I'd really like to be able to work with my students on supporting arguments part of the time, instead of spending every moment on style. For another, a lot of mistakes aren't as profound as "can able to." Some are editing errors, some are spelling errors, and some even the writer can't explain. While it's good to be able to approach grammar from a stylistic perspective and give writers choices, it seems like rather than actually spending all our time doing this, we can do it occasionally and help students to internalize the idea.
Lu, Min-Zhan. "Professing Multiculturalism: The Politics of Style in the Contact Zone." Representing the "Other": Basic Writing and the Teaching of Basic Writing. Eds. Bruce Horner and Min-Zhan Lu. Urbana:NCTE, 1999. 166-190.