Why not all of them at once? Give me six of each and reward me for the first 10. Maybe give me a small reward for completing more objectives, just like it is now.
Sorry, I didn't know that there was a blog post behind it. Is there a link behind your post?
If the selection only affects visibility i am not sure why we need this at all. The selection is only necessary if they dump all the objectives in one category. If they just sort them into different tabs: PvE, PvP, WvW you get all the dailies and a neat way to find the objectives you are looking for. You don't play PvP? Great you know that you don't have to look at the PvP tab.
From the sounds of the blog post, you can select as many of them at the same time as you want. It's just an option for people who, for example, never play PvP to not get their UI cluttered with PvP goals.
Edit: it might even go a step further and give you the same amount of goals, but spread only across the modes you selected, so everyone can complete the same amount of goals. Just speculation of course, we'll see on Tuesday.