Its not so much a case of adding extra grain because of BIAB has a low efficiency, its just that it is often recommended for novice brewers until they become familiar with their own equipment. Invariably with some first few attempts at BIAB there's an unexpected loss somewhere, be it extra trub loss, miscalculated volumes, poor draining, spillage, etc. and so a bit extra is suggested to take care of that.
When it comes to BIAB there's a few different variants, however the stock BIAB method (some folks may call it 'pure BIAB') isn't sparged in the usual manner as all of the mash and sparge water goes in with the grain at the beginning as a simple single infusion mash. Therefore it is different to a traditional mash which is then deliberately not sparged (as would usually be the case), where efficiency would most likely suffer. Another way to look at it is that, as far as water goes, stock BIAB combines the mash and sparge step into one, in which the mash is actually sparged, just that the sparge water is in there from the start.
Many years of experimentation and trialling on part of the BIAB pioneers have shown that this does indeed prove quite an acceptable method, and as per browndog's earlier
post, BYO seem to think it is now mainstream- that's probably quite a reasonable barometer. What often does come up though, and this isn't unique to BIAB, is that water chemistry is an important factor in ensuring a reliable and acceptable efficiency, however there's a few other factors as well, eg. Bribie mentions crush, mashout is another, there would be more.
To answer the OP's question, BIAB efficiency is usually quite respectable so no, there's really no need to add more grain. However in the novice brewer's initial stages of all- grain brewing, as I've outlined above, it isn't such a bad idea to be sure there is a decent yield and avoid major disappointment. More info linked below if you're interested.
