To me they are of very little use. They would attract attention if they go off on site, so would probably work there in most cases, but thefts from busy sites are rare. They can attract attention so it is possible they will attract attention in other locations but I am afraid the numbers seem to be very low.
Our caravan is kept in storage and on one visit I heard the alarm sounding in another caravan and told the wardens. The alarm was simply not loud enough to be heard outside the compound.
Our Avondale did have an alarm with the siren in the front locker facing through the vent but many thieves would know this and could easily disable it before it went off. Possibly one fitted by you in a more random place would be more difficult for thieves to locate.
They do give you a discount on your insurance so you may as well take that though.