Really, the question is:

Is it worth it to you to dual-box?

All of your questions are entirely subjective... Can it be fun in PvP - yes. Can it be fun in PvE - yes. Can it be effective in both/either - up to your skill level to determine that.

I started off in WoW 1.0 (4 years ago now?) with two accounts, two pcs and no idea how to broadcast keystrokes - so ended up with a healer on follow and a dps as main, that lasted me well into TBC when I tacked on another three accounts.