Destroying a Jump Gate would be like using WMD's (let's use nuclear for a sample): you use a nuke, you may destroy a large amount of opposing forces, but you create a Pyrrhic victory. You'd not only create logistical nightmares for generations to come by (possibly permanently) cutting off the route, but you'll also be destroying the ONLY route for ships to come, rendering the system "cut-off" from the rest of the universe, leaving it empty and useless. We may not realize it, but we're HEAVILY dependent on Jump Gates, and if we take even one out, it'll only do more harm than good to EVERYONE, not just those involved in the war.
Also, if RP had its way over game mechanics, Jump Holes wouldn't be the convenient modes of travel everyone knows and love: using one would be a gamble, since they're unstable, unreliable, and would sometimes send people off-course if it felt like it.
As for locking it down, tbh I have mixed opinions. It DOES make sense to try to at least make the two jump gates locked down, like when two countries declare economic embargo. On the other hand, doing so would force people to use jump holes, and "may" make FL a little more boring (albeit the longer transit times to get to your intended battle).
Edit: So yeah... it's all about the Rule of Cool, so that battles would be faster and more intense in a "game". Not everything has to be too realistic.