I wouldn't be too worried about how wet a tent would get rolled up, wrapped in nylon & strapped to the rack.
Rolling up a wet tent - if you're told to expect rain - then that is probably more relevant an issue.
Having said that, I use an Ortlieb rack bag to carry my tent, sleeping mat & fold up chair(!), as well as other bits'n'pieces depending on the trip & the season.
I use it for convenience, not for waterproofing properties, (but waterproofness is relevant because in pouring rain I do not have to open panniers & risk exposing "dry" items to rain to set up my tent).
For example, it also holds my first aid kit to be quickly accessible when I need it (no pockets on the Ortlieb panniers and my bar bag is for valuables). It also carried my laundry kit. (I've been known to give my clothes a quick wash in a bar bathroom).
If I'm catching a train/using a hotel etc. it's just easier to grab one bag instead of several smaller items.
Heavier, yes, but I like the convenience.
Previously I strapped my tent on the back of the rack and had no problems at all. Initially in the bag it came in, then when that ripped, in a drybag. Wasn't keen on using the drybag and wouldn't do it long term.
The picture attached is from a trip along the Rhine last year where temps varied from below freezing to mid 20's celcius.
Frank