I don't understand how people can still take the dragons seriously.... They spawn almost ever time you fast travel and go to dawnstar, which ruins their rareness and makes it repetitive, taking away a lot of how ferocious they are. After that amount of dragon appearances, people just spam attack the dragons as they can be pretty easy kills. When I see a dragon it's no longer "Crap Crap Crap, I need to hide and get out a good weapon!" it's now "Darn, another dragon; Oh well, easy soul"
Dragon riding will not cheapen the already cheap experience, if anything it can provide a new way to face the dragons (Aerial Combat). It's also actually in the lore as well. I can totally see where your coming from if you think the map may be a little small for dragons, but imagine if the dragons were at a pretty decent slow speed. I can trust Bethesda that if it's implemented that it will work pretty well. I personally would love to get a contract from someone saying "You really don't look like you can handle the job but if you can, there is a bandit camp north of markarth.." than just ride the dragon and bring flame to the dames or get revenge on that guy that says we don't go to the cloud district very often. If implemented right, it could work... and be extremely fun...
It's also very likely that like with the dawnguard DLC (The trailer said we could end the sun for good, and in reality we got a bow that shot arrows which would make the sky turn red for like a minute; kind ruined the roleplaying possibilities that could come out of a sun gone forever) , we will end up getting it as a fast travel option or something like that...