-Is the main advantage over extract the ability to use base malts?
This and you have total control over your recipe. You can successfully make a beer with an SRM of 3, which I understand you can't do with extract. I only made 2 extract batches, and went to BIAB, but the two extract batches I made had such pronounced maillard reactions since you're really using pre cooked wort and then cooking it again.. The colors were always way darker than what any program said, or what I expected..
being able to adjust your mash temp is probably the biggest though, although now you have to look at your water and ph more carefully.
-it seems I only need a large bag outside of my existing equipment (10 gallon brew kettle). Can anyone recommend the preferred bag?
I ordered one from a site called BIAB tailor and it works great, except I hate how the drawstring is made. But, he will make it to fit your kettle and I'm just going to fix how the drawstring is fixed myself. http://biabbags.webs.com/
-is there a chance that I'll get low efficiency and the extract process would have been better in this arena?
My mash efficiency is 82%, brewhouse 70%. You'll have a little learning curve getting used to your equipment at first. You volumes will be off, gravities, etc. After a few batches you should be able to get your equipment profile dialed in in a program like Beersmith and soon your numbers will be right on. I would advise either grinding your grain smaller or having you local shop grind it twice, which is what I do right now. I also don't do full volume mashes, I scale it down so my ph is lower and reserve the water to 'dunk sparge' with. Lastly, you'll get more proteins in your kettle with BIAB, so keep that in mind if you do a big beer. You might need to add a .5 gallon of pre boil volume for big beers, or maybe filter between the kettle and fermenter with something like a hop rocket or other device. My first beer with a 14.5 lb grain bill made a big mess trying to get it in the fermenter... Last advice, figure out a way to insulate your kettle. I have insulation I put on while mashing and take off during the boil. I don't have a false bottom and turn the heat off when mashing and I have maybe a 2degree loss over an hour, 3 for 90.
Even with that, it still takes less time to set up, to brew, and less to clean up compared to standard all grain without question. You should have most or all of your gear cleaned up and ready to go while the mash and boil are going.
Double crushing or a finer crush, plus the dunk sparge makes a big difference in efficiency.
You'll figure out your own way to do it and it's different for everyone. and yeah, extract is good to get your feet wet, but it's cost prohibitive when you look at grain cost, plus it's less flexible. I do it because it was a way to get into all grain without dropping another 500 bucks on a mash tun, HLT, burner. I just made a hot stick and do it all on my stovetop and the beer is great. Adjusting water and mashing in partial volumes made a big difference. Plus, like yesterday when it was 5 degrees outside, I was brewing an IIPA and watching some college hoops while mashing.
That's my advice. I only brew BIAB now. Do it, you'll wonder why you hadn't done it sooner.
edit: I also have a 10 gallon kettle. with how I mash and dunk sparge I do big beers with no problem at all. I dunk sparge in a hot water canning kettle I have. You grain bag will be wider than you think with a ten gallon kettle. You might do it in a cooler too, but those are a pain to clean and are too big, at least the ones I have.