^That is all true. You're right.
Anyway, when it comes to laser/particle-beam sail technology, there's a significant problem with one thing. If it is to be used as a viable method of transportation from point A to B, you're gonna have to have the same kind of technology at your destination, as a means of decelerating from the speeds you reach, otherwise you'd just zoom pass the destination and keep on going... And of course, to do that, you'd first have to get to your destination by some other way, and then build the laser/particle cannon there. Which kinda defeats the purpose of a sail-ship.
There's a proposed braking trick where you use an electromagnetic field to slam into the solar wind of the target star, a bit like re-entering a planet's atmosphere, except you're starting the deceleration out about where our Voyager probes are now. The technique should produce detectable Bremmstrahlung radiation, which is something to listen for in other star systems.