That is my (and I assume the OP's) point.
I'd still like to take some of my characters that haven't triggered dragons through the DG questline(s).
Yeah, I personally do not like playing as a vampire when there are dragons about, since they pretty much one-shoot you if you're not at stage one.
Traveling the world a bit can mean walking around. I mean you can pick up the shouts in the base game without even doing the main game why would it be different for Dawnguard.
Yes, it is possible to acquire shouts in vanilla without starting the MQ, but there is a possibility that they may block access to the Dawnguard questline if you haven't at least started the MQ and have dragons start appearing as regular enemies. There is a possibility though that the dragons in Dawnguard will be there regardless of whether you start DG's MQ, however.
But that means you've now triggered Skyrim's answer to Oblivion gates. If you just kill the first one, then attacks are reasonably rare, but those random attacks will happen. I purposely don't progress that far for nearly all my games, just to avoid those stupid attacks, , and it's annoying how many quest series require at least some of the MQ to be done, even where there's no real reason for it. The fact that it's intended for experienced characters is standard (it's true of all Beth DLCs), but they don't usually force you to do the MQ.
With the exception of the aforementioned KOTN for Oblivion. But the DLC usually doesn't make much sense for people who haven't done the MQ, with the worst offender being Tribunal for Morrowind.