I think they are pointless. Traditional radars, which are rare these days, even back in the day did not require a warm-up... once detectors became common cops just started leaving their radars off until they wanted to ping a vehicle. These days ladars are so common, and from what I can tell if your detector is picking it up, you've already been tagged.
-JV Xbox LIVE!: "Vlad is Rad"
PSN: "Vlad_is_Rad"
SCII: "Volodya" (code: 314)