Interesting! I don't entirely see the point of a rolltop on a backpack that isn't waterproof, because the main advantage of a rolltop opening IMO is that it allows for a very waterproof closure that is simple to design around, especially in a lighter bag, and without having to use big pricey water sealed zippers. Rolltops are kind of a pain to use, especially when the roll top is only intended to be closed in one way, as here. (Roll tops can also be designed to serve as a flap closure or clipped down on the sides, which makes them less annoying.)
Ostensibly, the roll top allows for expandability, but the added usable capacity is often not that big and since it's achieved by adding height to the top of the bag it often makes the bag unwieldy when you overload it. Also for a cycling bag it can impede visibility.
But increasingly roll tops are used just as a design element which really bugs me because you end up with a less usable product. If you want your bag to look cool, just make it fun colors (Like Cotopaxi or Topo Design) or weird shapes (like côte&ciel), don't make it hard to open for no functional reason.
I think the approach used on the Mission Workshop Rambler and Vandal is more useful with fewer tradeoffs. (Those are even more pricey than a Tom Bihn bag though, to be fair.)
Since the bag is also offered in X-Pac, this also brings me to one of my other grouses about not this bag specifically but many bags made these days: why offer a bag in waterproof materials without sealing seams?