Are you familiar with the musical terms "accelerando", "ritard" and "rubato"? Real-live breathing musicians with a pulse -- i.e. not computers -- know them well. If you don't, I'd look them up.
Otherwise, the use this of proposed plugin could be severely limited.
Yes I'm familiar with those terms, and appreciate what you're saying, but I don't think this would 'severely limit' the use of a beat/timeline grid.
There's plenty of music out there that does have a consistent tempo (particularly electronic music, and library music which is usually produced on computer), and there are also plenty of occasions when it would be useful to have a time-interval grid, even when music isn't being used. (I've started thinking of it as a 'timeline grid' rather than a 'beat grid' now.
What I'm proposing is not a plugin, but a simple addition to the user interface - a grid you can turn on and off, just like the grid in the comp viewer.
Personally, with the work I do, I would benefit from such a feature like this on an almost daily basis, hence why I'm putting the idea out there in the hope that other people agree.