For a simple reason... you have to face two Revered dragons in the Forgotten Vale.

You're basically forced to take a break from the questline after reading the scrolls at the Ancestor Glade.

Also, the Dawnguard armor... its armor rating is on the same level as steel armor.

If you're the role-playing type and you'd like to wear it when doing DG quests... you can't. It's only good for earlier levels, unless you resort to smithing and enchanting.

What level do you want it then? 20-30? what about the people that want to try it straight away? 10 is a good level and they are not forcing you to do the questline instantly anyways.

...and you can leave it unchecked in Data Files until you feel you're ready.

And to add what Over-Kill said, you can actually avoid the vampire attacks. So far I'm 24 hours in and I've only had an attack in Rorikstead.

I don't like how Dawnguard or Dragonborn handled the starting of their questlines. I really dislike having these things try and take center stage when I'm in the middle of a quest. It'd have been nice to be able to check/uncheck the quest triggers on the consoles, so that I wasn't running away from Cultists like I'm Benny Hill.

Having both DLC installed quickly ruins any sense of pacing or flow in the game for me, with Dragons, Cultists, and Vampires all banging on my door trying to get more screen time.

The DLCs should be triggered by an event in the game that the Player Character has to act on before the DLC events get started.

In other words, Posters appear for Joining the Dawngaurd and maybe a Vampire confronts the Player Character. From here an invitation is extended to go someplace. Once that is done, then the quest events trigger. However before that, no in town attacks to force the player into the quest line.

Same could be done for Dragonborne. Although, that one is easier to ignore until the character is ready.

If you don't start the main quest the dragons don't show up in the Forgotten Vale. No other enemies take their spot so you can just walk across the lake.