A mechanic who is used to servicing cars day-in and day-out might well not need his diagnostic equipment for most common faults. But for someone who doesn't have the same experience, an OBDII reader (approx £25 on eBay and even less if you already have a laptop) will be a useful tool if the engine check light does come on.
I don't disagree that for a coil-pack induced misfire then you can easily find out which coil pack is at fault by a quick bit of experimentation, but how many times does someone post up the details of a fault when the initial response from most here is "what codes are coming up ?" ?
As for saying it's the quickest method though, come on ... Plugging injectors in and out and then waiting 2 minutes with the battery disconnected, compared to plugging in a reader, reading the codes and resetting the MIL light ? :winky:
How about the guy who posted today with a P1126 code, you're saying you would diagnose that without simply reading the code ?