A little bit of weather never hurt anyone... it's even possible that you're more concerned about the well-being of your gear.
If you want to protect your pack from rain, snow & sleet - just throw on Osprey's High Visibility Rain cover. It has reflective graphics, which means it's perfect for night time travellers who need to be seen.
A blinker light attachment point offers extra visibility in low light for urban bike riders or commuters. It also packs up into it's own carrying case.