I'm getting terribly worried, as well. The senior level people in my company are extremely polished and I know they will come through with solid LORs. However, my direct supervisor has poor grammar and writing style, and he has explicitly told me that he plans to "take care of them quickly and turn them around fast", which most likely means he won't spend more than 15-30 minutes on the LOR.
Anyone else in such a predicament? If I were to not use my direct supervisor, it would set off red flags and I would have to explain it. If I use him (which is the choice that I have decided on), I may end up with 1 mediocare at best recommendation and 1 glowing one from a senior guy. They will likely provide a pretty stark contrast as far as content (one being "realistic" and one "overly optimistic) and writing style (one messy and one polished). I have no idea how this is received, but I would hope that it would reflect more on the recommender than the applicant.
I really hope that these don't count for much. I have never heard of anyone being told in a feedback session that their LORs are why they got dinged, and most books on the admission process seem to rank the importance of LORs fairly low. It is truly not my fault if someone can't write and/or if they are a negative person.