In my experience, most well-padded bags will hold dry in medium rain even for several hours. They will get soaked, but very, very slowly. The only source of immediate leaking could be the zippers. So the advise to the cautious would be to buy well-padded bags with weather-proofed zippers.

Note that it also depends on the design of the bag - e.g. mine has a separated non-photo-equipment compartment on the upper side (I use it for water bottles, pens, business cards,....), so even if some rain would enter there, I do not have to fear that my camera gets wet immediately.

Personally, I do not even know where I left the rain cover of my bag - in monsoon-type storms, I typically try to stay indoors or at least under some roof, and for everything else, I never even saw water getting in.

You could also try to further enhance the weatherproofness of your bag with an impregnation spray. They work better than nothing for all kinds of shoes, so it would be worth a try.

Note also that even non-"waterproofed" cameras will typically not die on you at the first exposure to the elements (i.e. a single rain drop), so 5 minutes of light rain should not even damage your camera in the open.