Yup, hardly worth doing IMO.
Insurance companies will demand them on stuff (Jap imports, Scoobs, Evos etc) but they have no affect on the price of your quote, just if they will or wont cover you.
According to the helpful person I spoke to at Tesco Insurance when I queried adding one to my previous car - they give you the discount on a single piece of security regardless of if its your immobiliser or a tracker, they still work out at the same discount and once only. You can put as much security on it as you like but it wont actually reduce your premium. Also as I believe its no wise to advertise that you have a tracker with a window sticker (makes it easy for a learned thief to find and remove?) you get no reduction in potential theft either.
My view on them is, I get to pay £400 for the unit plus a subscription of X amount. I then get the joy of having my car back after some junkie has ragged the pants off it or jumped up and down kerbs with it. Insurance company wins because you have no payout on the car to claim.
I would rather my car gets nicked and I go buy another one with the payout.