I actually have two (if you want to really get into the reasoning of why that particular antenna is installed where it is)…the other was on my Trailblazer, on the roof. One night I was coming down a dimly lit city street and hit the antenna a little more than half way up on a low hanging tree branch traveling roughly 15 MPH, which is frankly slow to me. By then, the damage was done. The antenna had actually pulled the NMO mount out of the roof of the vehicle.
Notice now, Comet sells a spring for that antenna. I guarantee you I'm not the first one who has pointed out to them (out of first hand experience) that the antenna they market as a Search and Rescue antenna, is way too stiff to be useful in a real Search and Rescue environment. They may have also been looking like they were facing several lawsuits.
Back to the antenna being on my Trailblazer. While it was on the roof, I actually took the time to hook the antenna up to an analyzer and the results I got, are what I quoted in my previous post.
Now back to where that antenna happens to be on my Jeep. I'm not sure if you've ever spent a lot of time in heavily wooded areas, especially driving vehicles through them, but you're always gonna hit antennas on something. After witnessing what happened to the roof of my daily driver, I carefully picked a mounting location that would both be sturdy enough for that antenna (until I found a different antenna solution that would suit my bandwidth needs). So I chose to sacrifice some signal to mount the antenna on something much more sturdy than the roof. I might also add, the only current use for that VHF 1/4 wave antenna on the roof of that Jeep, is for an APRS tracker I have in that Jeep.
Here is another interesting little tit bit to look at. A quarter wave antenna (VHF or UHF) cost roughly $5-12 (or I can even get a Larsen NMO150WB for $30). My wideband UHF knobs, $30. The last CA2x4SR I bought cost roughly $75. However, if roof mounted, the CA2x4SR can cost you several hundred dollars at a body shop if one was to hit the antenna on something in the right spot. For the other options, you'd be worrying about dragging the roof before you ever even considered the antennas doing damage to the roof from snagging on something.
That is the primary difference between commercial antennas and amateur antennas. Commercial antennas are designed to take a beating through a parking garage, in the woods, over rough terrain, even survive a roll over. You break a base, you can usually buy a base without buying a new antenna. You snap a whip, you can generally get another for less than $10. You just don't get that kind of quality out of an amateur antenna, and you definitely don't get that out of the 2x4SR.