Here's my analogy to this: The chances of someone breaking into my house in the middle of the night is relatively small. Yes, my Mossberg is a little heavy, and I could save some weight by replacing it with a can of pepper spray, and call the cops (and wait for them to show up) in the case of a break in during the middle of the night.
Buuuutt... I personally feel better knowing that I can grab the "heavy" Mossberg (insert favorite shotgun/rifle/handgun or combo thereof) and be better equipped to handle the situation immediately rather than hope the pepper spray is effective and the cops are quick. YMMV