Only if it is a close combat unit like zergling, zealot, etc I think. If they were long ranged unit like hydra, then cannons can't reach them unless they actually walk into the mineral line.
yeah, and against a T you have to be very carefull, or your main is gone when T pushes to your choke. dunno if this realy works out ingame. only meelee units are affectd anyway.
the problem is that when you are being delayed expanding because you dont have cannons in pvz, it generally isnt becuase you need the cannons to scare the lings away.
on top of that, with the way it is placed, zerglings can still hit the nexus from the far side
and yeah the pvt thing that listoric pointed out can be a really big problem, because quick pushes and contains in the early game would be very very powerful.
the idea of the cannons protecting multiple points of interest is interesting though, such as placing a cannon where it could protect both a players ramp and their nat choke, but the problem with it is that those sorts of areas favor terran and are generally easy to exploit as terran.
vs. mutal harass and vulture attacks you generally wouldnt want the cannons located there because they cover less of an effective area.
generally you want the cannons to protect the nexus, the assimilator, the miners, and the cannons around it, and the placement of cannons that lead to this is generally located near the nexus, not behind the mineral line. when its behind the mineral, youll have no coverage of the nexus (at least vs ranged units) or the assimilator, leaving you only with protection of the main mineral works (and even then not as thorough as better placenment would be) and even more limited protection of the natural workers.