With sparging you are effectively diluting your first runnings so to get back to your required volume you need to boil off the additional liquid.
Not true.
If you use the full volume of batch water for BIAB and drain completely you are leaving a very diluted wort remaining in the grain and since you can drain the grain (more or less) completely you are emulating what sparge brewers do using any mash method.
For example a sparge brewer might collect first runnings of 1.085 and add the second runnings of 1.042. The combined pre-boil gravity might be 1.060 (I'm just throwing numbers out there.)
A full volume BIAB brewer might simply collect 1.060 wort.
Both are leaving a diluted wort in the grain and ending up with the desired boil volume.
Since BIAB brewers can usually crush finer without risk of stuck sparge, they are more likely to get a higher mash efficiency. And since they generally drain more completely, they also usually get a really high brewhouse efficiency. The take-away from this is that saying BIAB is less efficient is complete crap. it can actually be much more efficient if conversion happens as expected in a highly diluted mash.
The main differences between a no-sparge BIAB and other methods that use sparge is that you are mashing at a much higher water:grist ratio than convention calls for, and you are adding a lot of particulate to the Boil Kettle that might otherwise be filtered out.
Those differences might not mean anything to the brewer themselves, and anecdotal evidence seems to indicate that there isn't a noticeable difference in finished beer appearance of flavor between them.
Of course, there are many ways for all kinds of brewers to handle the mash and there are really not too many blanket statements to be made between them. Homebrewers have come up with various tweaks to the various mashing methods to fit their own preferences.