Here's why:
If you take all seven emergency vehicles to the crane in Portland, you get a nice cash bonus, AND seven icons on the ground. These icons let you "order" any of the emergency vehicles, free of charge -- the crane will lift one off the boat and drop it at your feet, and you can do this as many times as you like.
In other words, if you deliver one FBI Car to the crane, you can get AN INFINITE NUMBER OF RHINO TANKS slightly later in the game, with zero risk involved in obtaining them after the first one.
Reason enough?
(Without the Parking Garage method, the FBI Car's one of the hardest vehicles to obtain, because the FBI agents are nuts and heavily armed, and when you have five stars they come in bunches. The Rhino is actually easier, IMHO, because it's nigh-invulnerable -- you can use grenades or rockets to take out its guardians without worrying much about blowing it up. Only some of the bulletproof vehicles are harder to get; the bulletproof Cheetahs are a pain, and I've never managed the bulletproof Bobcat.)