Thing about what I underlined. If your leveling solo in the game with only the help the a companion you have no choice but to pick the Advance class that works best your first companion.
Like mako is healer she works best with a tank so I picked the tank advance class but when I get blizz my jawa tank companion later on the game I would of like to be a healer for him but I can't because I'm stuck with my advance class because I made the choice to pick what felt the most efficient route
Don't even give me the oh but you can't do that in WoW because I've got a druid that was dual spec tank and healer and I had both sets of gear for the specs.
I use to run randoms all the time on him.