I've got this saddlebag. It is just the right size for a change of clothes / lunch etc.
It also has the bonus that if you are a fashion victim who refuses mudguards, it makes you get a less wet back. Oh and is waterproof, which a lot of rucksacks aren't. And no sweaty back.
I have it because I ride my only road bike to work, and I don't want to have a rack on there when I go on fun rides. It has most of the advantages of panniers, without having to have a rack (although it won't take as much stuff as panniers). No sweaty back makes a real difference.
However, she's already selected a full-on road race bike over a hub-geared/disc-breaked/flat-barred commuter bike,
If I was doing 5000 miles a year*, I'd certainly want to do it on something light with drop bars, way more practical for that kind of distance than some horrible flat bar thing with no choice of position, and no aerodynamic position on the downhills – I found I take tons of extra time on my commute if I don't get low on the downhills.
Joe
*actually, looking at my riding log, it appears I am going to have done something like 5000 miles by the end of the year (and on a lightweight bike with drop bars)