I use Motorola mag mounts for my roof mount. It has a much stronger magnet that my local fellow hams' mounts have although they have not had a problem of their whips blowing off. Mag mounts add a bit of capacitance and will change the tuning of a vertical antenna. That said, most vertical mobile antennas have a pretty broad tuning range and will unlikely change performance. And, it only matters if the antenna is center mounted on the roof of a vehicle with a fairly large roof. I am thinking that even pickup trucks with small roofs are kind of a toss up about anything that could be called 360 degree uniform gain. Note that at RF a DC connection is not a requirement. Second, vertical mobile antennas with gain flattens the donut characteristic of 1/4 wave whips. For someone traveling in the country away from cities, I always specified gain antennas. Those in very urban areas with base station antennas on the top of buildings got the 1/4 wave whips so more of the gain pattern is more vetical and is rare for ham radio (my opinion & guess). Even the 1/4 wave whip on a car parked under a mountain top tower mounted base antenna can experience a noisy signal. Probably a rare occurance for most hams but interesting to test given the opportunity.