35x17 gearing gives a speed range of 6.4kph to 34kph assuming a pedalling cadence of 90 rpm, which is what I average in normal riding.
I very rarely ride a loaded tourer at 34kph on the flat, and tend not to pedal on fast downhills, so 35x17 would certainly be OK for me.
The only time I often want to pedal faster than 34kph is for large roundabouts, when I like to match the traffic speed as far as possible. I get round this by increasing cadence, at 110 rpm the top gear with 35x17 equates to about 45kph. I can also drop cadence to about 60 on very steep climbs, at 60 rpm bottom gear with 35x17 equates to about 4.3 kph, which is a walking speed.
The validity of this depends on your pedalling style, I "spin" with 150 mm cranks, but not as fast as Dan. Others prefer to "mash" at lower cadences, often with longer cranks, so need higher gears.