They are actually available in many more lengths than 8". They are also available in a variey of "gauges". I used relatively short, very thin ones to guide or bundle brake and derailleur cables when I was a bike mechanic. Where I work now, we use long heavy-duty zip ties to hang polycarbonate windshields on the forklifts. These are used to feed solid ingots into our melt furnaces. They withstand radiant heat pretty effectively, but do not fare so well when splashed by molten aluminum. Civilian and military police use plain or specially modified zip-ties as expedient restraints in situations where there are likely to be many arrests.
These are just a few "snapshots" of uses. I think zip-ties can be counted on to reliably perform many other tasks. That said, I'm not ready to ditch my 550 cord to make room for them. Paracord is just too versatile to be replaced. It can do nearly everything that zip-ties can, plus it can be cut to length, braided for strength, peeled for thinner line and is much easier to reuse.