This is true in Nightfall as well, however, in NF they do give you access to the newb island with your lvl 20. You can play through the early content if you want, or you can skip it.
My point about taking your chars from other campaigns over is for when you've already beaten the game once and want to bring your other characters over to get a particular item / try a particular farm build / or just have a lvl 20 of a particular class available to the guild in the new missions too. I really enjoy the ranger class, but I don't want to start a ranger from scratch each time an expansion comes out. I like my current ranger and my current ranger items already.
Another benefit of linking accounts is that you have access to all the skills from all the campaigns for all your chars. I have a few builds that are dependent on skills from multiple campaigns.