This folder can get VERY big. You can copy it to another hard disk or even a DVD (assumes it's not too big). In other words, be aware that a few thousand images could easily result in a thumbs folder that won't fit on a DVD.
For backup you also need to be looking at the Library.aglib file because it contains all your settings, metadata, etc. You can always rebuild the thumbs, but once the Library file is gone you're in deep smelly stuff.
Interesting, IView's database of thumbs, based on 800px wide previews, occupies, for me at least, 1.2Gb for 13000 images. Bigger previews are not really needed as it is a DAM not a browser but it can pull out a full size preview from online files quite well.
This is one area where I think LR falls short. It needs full DNG/RAW metadata compatibility so the Library file becomes somewhat redundant, at least for archived DNGs. All settings and metadata must be included in the file itself otherwise it is just asking for trouble to store the info elsewhere.
IView is the DAM of choice these days and if you synch the annotations with the originals, either RAW or DNG, then even if you lose IView's database file you can still rebuild from scratch quite easily. It takes about 2 hours to catalogue about 13000 images - no big deal.
I strongly disagree with any approach that separates any information about a file from the file itself. This is why LR can never be a decent DAM until it addresses this issue. I have said this before but I still think LR is trying to be too many things at once and so is doomed to be mediocre at each task, with the possible exception of the actual RAW processing which is excellent.
Are there any photographers out there who use LR as their sole tool of choice? Do people like Jeff Shewe and the other developers/testers really use LR in their work by choice or because they are heavily involved with LR's developement and therefore must be seen to be using it.
I have to say that if it wasn't for Jeff and co, whose opinions I respect greatly, I would not even be trying to like LR, it would be deleted as a 'close but no cigar' app.