I'd say that 17" is an important market, as shown by Epson's products - the "I'd just get 24 inch" misses the point that printers like the 3880 are about as big as many people want to fit in their offices/homes/budget.
This gap in Canon's range is an interesting one, and having chatted with lots of people from the consumer imaging and LF printing parts of Canon, I'm of the opinion that there is a bit of a cultural gap between the groups back in Japan, particularly when marketing are involved...
With the consumer print side we get the Pro-1 (and below) and a very different approach to marketing printers, compared to the large format side of the business, which is corporate sales minded and has a solid dealer network and supply chain that if not entirely set in aspic, doesn't show the flexibility you see in the 'consumer' chain (which includes pro camera kit) - people that sell photocopiers and office print solutions tend to have different customer and business relationships.
It's this corporate setup that puts the 5100 at the bottom of one range, and just outside another. Compare with Epson, where at one level there is a more homogeneous feel to the range (or at least the gap isn't so obvious).
Note, that I'm in the UK, so I'm just reflecting my own thoughts on what I've found in Europe ;-)