1) Jump gates can only give you the frequency identification of a
given ship some minutes prior to exiting hyperspace; just as it's about
to exit, you can get much more detailed information, such as silhouette,
mass, and so on. By then it's pretty much out...but at least you know as
soon as it's out what it is.

2) You can't just shut down a jump gate like a light bulb. It is a
VERY powerful piece of equipment, and it takes a long time to shut down and
a long time to start up again. It's like a nuclear or fusion reactor in
that respect. If you shut it down, it'll *stay* down for some time, which
may put you in a very bad position if you have to leave fast, and that's
the only way out.

(During the Earth/Minbari war, jump gates created by the Earth
Alliance were programmed only to accept certain coded frequenies that were
changed regularly. [That should read frequencies.] This helped prevent
Minbari ships using EA gates.)

3) Really large ship, such as war cruisers and long-range explorer
vessels are powerful enough to punch their own entance into hyperspace,
creating a jump point. (Something you'll see happen in our first
episode, btw.) They can go in and out of hyperspace on their own, so
they don't strictly need a gate, which is primarily a) for smaller craft
incapable of generating their own jump point, and b) to help larger craft
conserve energy. The Vorlon fleet could have come in via its own jump
point, but the gate was there, and it allowed them to separate their
smaller attack fleet while in hyperspace, so they could all come out
together, as opposed to releasing them after making their own jump point.

jms