I'd have to - strangely perhaps - agree with both Bushflyr and bdcochran on this. In the woods, a good tomahawk can be very effective; for defensive use, it would be far from my first choice. In the woods I'd prefer a normal hatchet over a tomahawk, but to be fair a tomahawk can still accomplish quite a bit.
That said, epsecially seeing as you already have a combat-oriented tomahawk, I could never see a 'need' for a second; but it doesn't always have to be about need. Sometimes "just because" is a good enough reason for a new purchase.
At least for me personally; my wife seems to feel differently about that. She didn't at all buy the "some of them are left-handed knives and some of them are right-handed knives" argument when she looked in the safe a while back.