A soft bag on your roof rack is a bad idea unless you protect it with plastic or something to keep it from getting soaked, frozen, or caked with road grime (salt, dirt, etc).
You would think this but it's honestly not an issue. Although the back and sides of our car get completely caked with road grime in winter driving conditions the stuff on top doesn't get much dirt on it at all.
OP, I've got a Subaru Outback that we have a cargo basket (Yakima LoadWarrior) for but no hard carrier or snowboard carrier. We regularly strap 4-5 boards/sets of skis onto the cargo basket for ski trips (2.5-3 hours each way depending on the mountain we go to) and they stay clean and have not shown any sign of wear. It's not even loud. I think we've done 3 trips with them on top of our car this way, and one trip with them on the same basket on a friends car (Jeep Grand Cherokee), so over 1000 miles driven with the bags on top of the car so far this season.
It even poured rain for the entirety of one return trip (3 hours) and very little got into the bags and they were none the worse for it. We've used several types of bags (REI ski bag, Dakine Tour, Dakine Low Roller, couple of generic bags owned by friends) and none seem to have any more issues than others.
We like this solution because it makes use of stuff we already owned and allows us to store more than just the boards/skis on the roof. The downside is that it's a bit harder to get stuff on and off the car than a cargo box would be, you can't lock it easily, and the gas mileage hit is relatively large (though probably not much worse than other car-top options).
Here's a pic of the car with four boards and a set of skis (in bags) on top of it: