The developers of some of the hands-down best Illustrator plug-ins out there, Astute Graphics, certainly seem to think symbols help manage file size in vector-heavy files:
"Vector artworks can contain multiple objects or groups of objects resulting from duplication. In this case, save the item that you want to duplicate as a symbol in the Symbols panel (Window > Symbols). Then proceed to the duplication character, for example, using the Symbol Sprayer Tool. Using symbols greatly reduce the size of the original file. Their use is particularly justified if you want to duplicate complex objects."
From: 9 ways to minimize file size adobe illustrator
But in that same article they give a lot of other useful filesize tips (worth a read) including one I know helps a lot: uncheck the PDF-compatible file checkbox when saving your AI, and check compression - this will save you in many cases >50% file size. If it's required that you save a PDF compatible AI later for collaboration, you can do that in a separate save-as - work compressed as much as you can and it will help.
I have to work in huge illustrator files frequently when working for some architecture firms (100+ Mb illustrated site plans) and can say that once they get to be unwieldy, I tend to work in outline mode most of the time for that same reason - no point waiting for rendering of complex transparency and shadow effects on the trees as I scan and pan, if I already know they're correct and now I'm working in another layer adding the hardscape or entourage.