some other factors to consider:
-in terms of weight, a set of waterproof bike bags are not really any diff than wet-dry bags, my Ortlieb rollers with the thinner material are not heavy at all.
-baggage stability, with just wet dry bags, you get into how to attach them securely to a bike. I have had a"regular" bike bag come partly off a few times on fast downhills when hitting a bad bump, Ortlieb and other brands have a system that keeps the latch system on your rack and avoids this. I slap my bags on and they stay there, its a no-brainer.
-cost over time vs convienence--yes wet dry bags will have many other outdoor uses, hiking, canoeing, motorcycling--so may well be worth it more for you to have a set that will do many things. I personally am a life long biker, commuter, so spending more on a set of waterproof bags such as Ortlieb was worth it for me in the long run. I use them on day trips, longer trips, commuting, I put groceries in them, whatever. I just grab them and mine have lasted I think 17 years of nearly daily summertime use, a pretty good life expectancy.
no matter what, waterproof stuff is great. I wouldnt want to go back to non-waterproof bike bags again, just for the convienence.