How long are we talking on those timers? A couple of hours at the most, sure. Special conditions are more interesting though.

The problem with super long timers is that it doesn't really foster a positive server community. Right in your post you mention enemy linkshells. When you've got really long timers you get claim wars, you get people attempting to sabotage the runs of others, it breeds animosity and hatred for anyone who comes along that might "steal" your claim. It might be a thrill for some but I don't think that really justifies the rest of it.

I'd prefer to see hard NMs as opposed to ones that simply don't spawn very often. It doesn't matter if the NM is up all the time, if it's very hard to kill then you still get to have the "rush" of winning when others can't.