They are easier and more useful since they do not expire. In andromeda they expire in 26 turns if memory serves. Perhaps this has changed but it might not.
As to the other question, it is all up to you. Do you want anybody who travels through to know that you have been there? Will the beacon actually deter anything? Do you expect your neighbors to agree with your claim to the system or is this just a way of meeting your neighbors?
I have not found them too valuable myself, but others have. I would find them more valuable if they at least would tell you a fleet moved past. As it sits, the only info you get from them is when they cease to exist, and then you have to notice that they are no longer there.