Dutch cycle paths and tracks are usually smooth enough for lower hooks to be unnecessary, apparently, so many bags come without these.
IME the usual issue with Dutch paths is higher frequency vibration of the type that rattles nuts loose, rather than unshipping loads. That's as a Brit on a German tourer with air in my tyres, I assume the authentic Dutch clunker running at 15PSI (or the metric equivalent) smooths that out quite nicely, or has at least shed all its loose nuts years ago and is making do with structural corrosion.
Personally I'm of the opinion that while low-faff luggage can make a good deal of sense, I'd still rather it was attached securely to the bike. Baskets fixed properly in place beat panniers held on by gravity and the absence of potholes. It's hard to beat the Ortlieb system for convenience and robust attachment, other than by one of the proprietary locking systems (R&K, Racktime, Topeak).