there are seversal factors where a downrigger may become useful (such as taking your lure to a certain depth where fish are feeding) also if you are trolling at higher speeds it will keep the lure sub surface (a heavy lure wont be at any great depth when trolled fast without a downrigger)
e.g even with lets say 100 foot of cable out on the rigger your "real" depth will depend on your speed (its quite an experience to see an arched rod suddenly spring up to slam over with line screaming off the reel)
I always enjoyed trolling live baits for hoo but you can cover a hell of alot more water if you troll faster with lures
TRy googling wahoo and downrigger for more info
http://hometown.aol.com/waiten4thebite/wahoo.html