Can someone explain to me why swimmin a jig would be preferable to using a spinner bait?  When do you use one over the other?

One advantage is that you can swim a jig through places you wouldn't dare a spinnerbait.

Like where?

Yeah where Will? I am pretty daring with a spinnerbait. ;)I think it is a experiment kinda deal. Sometimes if I see a lot of guys slinging a spinnerbait I might change to a jig. A jig is not going to spook fish as much as a spinnerbait crashing into the water. Sometimes if the water is clear a jig might be a better choice for super shallow fish in areas that are weedy.

I'll definately throw a jig as deep into trouble as I would a SB, so that's not a reason for me.

I swin a jig slower than I retrieve a s/b. Pretty close to "slow-rolling" a s/b but the appeal is a "craw" look instead of a "baitfish" look.

Jig also has faster fall rate

I often hear of times when a blade might actually spook the fish, if that's true that might help right?

My theory on swimming a jig:  its a presentation not normally seen in a different zone.  Swimming a jig has been around way before the hype. Ever fish deep docks and watch the jig fall back (swim) to the boat.  Its not normal swimming jigs thru  schooling fish, but a white jig swam thru is a different look, approach, technique.  Now a jig is not only used for thick cover but open water also.  This time of year I like a white jig with a white grub as my trailer, ribbon tail gives it more action and vibrations.

Like others have said, its a totally different presentaion.  Sometimes it is the really key technique.   I catch more fish swimming a jig along docks in sunny, protected coves in the prespawn than with spinnerbaits but I have more luck with spinnerbaits in other times of the year.  Its a finesse version of the spinnerbait.  Try it when fish are active but they dont want a spinnerbait.  Just play around a little bit.  Find what works for you.