Well, they are a business entity and the primary purpose of a business entity is not just about serving the public with goods but also generating profits & revenue to ensure smooth business operations.
There are always techno-wannabes, enthusiasts, hobbyists, working professional photographers. I believe the purpose of seperating the S-IS, G-series are geared towards different market segments since personally, I don't recall a super 10x zoom lens with excellent photo quality frm 1x all the way to 10x.
The G-series, I believe are more for a niche market whose next camera would be a DSLR. Kind of like a step-board onto (D)SLRs. Since I guess 'The Power of 3x Zoom' is not to be sniffed at. The best lenses after all, rarely goes above 4x. Lenses like the 17-40L, 28-70L, 70-200L, etc. Rarely go beyond the 4x factor.
The S-IS series, I believe are for ppl who are looking for an above average camera beyond the ixus series with more advanced features & additional focal length but not willing to spend like $400 for a 480EX speedlite?
Pro1... Well, I personally not sure where exactly does this weird baby stand. But den I'd like to see the Pro1 more of a convienent alternative to a (D)SLR minus the bulk but with the quality.
Of cos, the intent is good, a 'in-between stepboard' for every level up. More profits for Canon. More choices for the average consumers, but for above average consumers who knows what they want and wish for certain stuff... well it'd prolly mean a disappointment for them.