Well, there is usually a good amount of debate as to whether that horizontally mounted shock, with its very limited amount of travel, really does much anyway. So performance-wise it is debateable whether it even matters.
I guess your picture failed to attach, so I can't see the condition or the grommet, but the one thought I would have is that if the grommet were completely gone, then the shock would hammer against the mounting stud every time you hit a bump, which could, over time result in breaking the stud loose, and then you have a shock flopping around loose. Granted, this is worst case scenario. So a lot depends on the condition of the rubber (maybe try posting the picture again), and how long you expect to go before having it in for service.