Although I still haven't bothered to check and see how well Word actually catches these errors, they are clearly grammatical errors and not just failures to connote intention correctly. Their and they're occupy different parts of speech -- they are not interchangeable. In particular, any sentence that swaps them pretty much automatically has an invalid number of verbs in it, because it either has one too many (from an incorrect you're) or one too few (from missing the are part of you're). So with these two particular errors, it is not possible (at least not outside of very contrived circumstances) to make a syntactically valid sentence in any meaningful sense that contains these errors.
I have no argument with the theoretical point, but practice is much less satisfying. At least for the OS X grammar checker, it seems fairly easy to trip up:
There it goes again.
Their it goes again.
They're it goes again.
(All of these are judged to be correct. In fact, I'm having a hard time finding a misuse of "there" and friends the OS X checker
won't accept.)
Your dog tired.
You're dog tired.
(The grammar checker gets it exactly wrong. The first is grammatically correct, but there is almost no case where an actual English speaker would ever use it. The second is correct, but "you're" is marked.)
The MS Word grammar checker is somewhat smarter. It recognizes "their it…" as incorrect, but not "they're it…," which is odd because the latter is more obviously wrong. "Their…" at least has some syntactic credibility, though little semantic. In "you're…" it helpfully suggests that "dog-tired" should be hyphenated thusly, which is technically correct, but common English has a great tolerance, even preference, for dropped hyphens. Hardly anyone would notice or care if the writer omitted it, particularly as compared with a "your/you're" violation. Further, it marks "dog tired"
only in the "you're" case. On the other hand, it marks "your" incorrect without considering the edge case that I might be stiltedly discussing why your dog lost a race.
Arguably worse, the OS X grammar checker marks several things in the above paragraph that I know are correct, and missed one error I caught in proofreading. Those who cannot recognize when the grammar checker is wrong are doomed to overlooked errors and awkward rewrites that may not preserve their intended connotations, all to satisfy a machine that cannot actually
read anything.
Then there's the confusion a grammatically challenged writer would experience just from the fact OS X and MS Word have conflicting "opinions" of what constitutes acceptable grammar. Anyone would become frustrated trying to rely solely on the machine to judge their grammar.
I stand by my statement that the checker is a crutch only. It can be useful in many cases, but it is no substitute for knowing what you're doing in the first place.