I don't think building fixed defenses at this stage of the game is worth the effort. We should instead be investing in the resources necessary to develop flexible fleet units capable of dealing with more than one event scenario. We cannot know where all jump points enter the system, so wasting resources on fixed locations in space...rather than fixed locations like bases, is too restrictive, IMO.
If we want to build fixed defenses, they should be priorities at strategic bases, rather than strategic points. If the point is strategic enough, we should build a base nearby.
We won't be deploying any Jump-Point fortresses until we've located the jump points, naturally. But once located, the jump points are very strategic locations indeed. Not only do they represent bottle-necks through which the great majority of our potential enemies must pass in order to reach us, there is also jump-shock to be considered. Ships are almost helpless for anywhere from two to five minutes after a jump... it's the ideal time to smash them at short range, while they can't even target you in return. Naturally, in order to smash them "at short range", you need to be deployed practically on top of the jump point BEFORE the enemy comes through. That's where the towed fortresses come in... it would be wasteful to use our mobile fleet in this role.