Yeah, the Cortez's engine were hit and they could neither keep themselves on the beacon or jump to normal space. The danger of drifting too far off the beacon in this case was that no rescue ship would even be able to locate them. Even if they got the sublight engines going again but were still off the beacon they'd likely never find the beacon network again, or not for a few hundred thousand or million years and would be stranded in hyperspace.
If you did manage to fix the jump drive and get back to normal space then you're still in the middle on nowhere. Your options are to make a dash for the nearest jumpgate at near relativistic speeds and hope your grandchilden can learn how to fly the ship once the current crew has died of old age or start doing random jump until you come across a system with a jumpgate beacon. The former option is more likely to work since, well...space is big. Really big. You just won't believe how vastly hugely mindboggingly big it is. I mean you may think it's a long way down the road to the chemist, but that's just peanuts to space...ahem. Sorry I came over all Peter Jones for a moment there.
Anyway, while you're in hyperspace, navigating without a beacon is (for most races) impossible. You may think you're going the normal space equivalent of 30ly on a heading straight for Orion but when you jump you'll probably find yourself 70ly in the opposite direction, facing the way you think you just came. It really would be random. At least in normal space you have a clue where you're going and will get there eventually.
Plus of course random jumping means you run the slight risk of jumping right into a star, a planet, black hole or other equally dangerous stellar object.
As for the beacons, I think we've had this discussion before and I still think the most sensible way for explorers to work is to send out probes via normal space at near relativistic speeds with their own tachyon beacons that a ship can follow is the probe finds anything interesting.