These antennas were designed with the trucker in mind, and are great for the highway because they are stiff and stay more vertical than steel antennas at highway speeds, and work good clamped to the mirror frame, where they can pivot down out of the way if they hit something. However, for off-road use these antennas present some problems.

Durability is at the top of the list. Where overhead obstructions are common (like here in Washington state), a fiberglass antenna is a liability. They are brittle (especially in the winter), and it is not uncommon to have them shatter when they strike an obstacle. Once they crack, they usually break the wire winding inside, and can damage your radio if you transmit with it. They don't even need to crack to break. If they take a hard hit, or scrape against rocks enough, the wire can become damaged, and again cause radio failure. The bad thing is, you don't always know when they are damaged, and keep using  it anyway.... then it's too late. Unless you keep an SWR meter in-line all the time, you would never know until you fried your radio. People have even fried them twice, because they didn't know what caused the radio to fail the first time, and hooked up the repaired radio to the same damaged antenna. Only then did they suspect the antenna. (I always do, and ask my customers to be sure it's okay).
 

Another consideration is the weight of a fiberglass antenna. They are heavier, and not as flexible as a steel antenna, and because of this, they are very hard on their mounting system. They need to be mounted on a heavy duty ball mount, (or tire carrier - see Rack Mounts). A front fender bracket mount is not up to task of holding on to a fiberglass antenna under the harsh conditions of off-road use.  The vibrations and the swinging momentum forces of the stiff and heavy fiberglass antenna will cause stress to the mounting surface, and the surrounding area, and in time will result in stress cracks and fractures around the mount, as well as the continual  loosening of the hardware. A spring here can actually make things worse, allowing the heavy "baseball bat" to swing back & forth more violently.
 

Technically speaking, the wound fiberglass antennas (like Firestick) are an attempt to fool the radio into thinking it has bigger antenna attached to it. They boast of having a full 1/4 wave (or more) of antenna wound around the shaft, to give you the effects of a large antenna in a smaller package. Well this is fine in theory, because you are providing the radio with the required length of antenna wire, and the radio has the 50 ohm load it needs to perform correctly (i.e.. transmitting full power into a resonant load). But how the antenna really uses that power is another matter. It will only be as good as it's overall length. In order for a 1/4 wave antenna to perform like a 1/4 wave antenna, it needs to be 102" long. Anything shorter, will not perform as good. Heck... a 50 ohm resistor will provide a perfect load for the radio, but will be worthless as an antenna. The Firestick is somewhere in between a 50 ohm resistor, and a 1/4 wave antenna.

Don't get me wrong, I'm not trying to knock Firestick. Their antennas work fine. They just aren't as reliable as a commercial grade steel antenna (few consumer grade antennas are). And since they don't out perform a good quality commercial antenna like an Antenex C27  or a Maxrad MLB series 27, why put up with all the shortcomings of the fiberglass?  And if you're going to put a ball mount on your rig anyway, why not have the ultimate radiating element on it, a bullet proof 1/4 wave stainless steel whip?

A steel antenna being more flexible, will be much easier on the mounting surface, there are no wires to break, and no matter how hard you hit something with it, it will not break, only bend, and is easily straightened  by hand.  And in the event it gets so twisted up over time, that you can't stand the sight of it, a new whip for an Antenex or Maxrad is only about 10 bucks.  Another benefit of the steel antenna is that it can easily be bent over and secured to the rain gutter with a clip to keep it down and out of the way when not in use, or when parking in a low overhead garage.  They do have a tendency to "whip" around, but this action can be limited by the use of a tether which can be  made from light nylon cord or heavy fishing line (more details on request). This is a good idea if there will be "spoters" standing near your vehicle as you pass.

You would never have to replace a 102" stainless whip, unless you lost it.  Somebody didn't tighten their 1/4 wave into the ball mount enough, and I found it laying on an old trail, in perfect condition, after it had worked it's self loose and fallen off spring and all. That was 4 vehicles, and 20 years ago, and countless encounters with low flying obstacle, and it still works (and looks) great. Nobody has come along to claim it yet, so 'till then, I'll keep moving it from one vehicle to next as I wear them out.