B12 needs IF so it can be naturally absorbed. Given together (B12+IF) in a dietary supplement, the formulation supports the special dietary needs of people who can't produce adequate IF themselves.
The supplement industry has become accustomed to putting very large doses into their formulations because it is well known that B12 absorption from these large doses is only about 1% and not through the natural active transport of the B12 receptor. [2,3]
The amount of B12 not absorbed is not simply expelled in the bowel, it is broken down into small fragments called 'analogues' that get used by bacteria in the colon and can contribute to bacterial overgrowth and Irritable Bowel Syndrome. [4-7]
Each B12 molecule can produce dozens of 'analogue' fragments which can be absorbed and are not metabolically active. In fact, some analogues have been shown to interfere with B12 function in the cells. 
333,667 people were followed for 10 years and 22,652 (7%) were diagnosed with cancer and were more likely to have higher B12 levels, even though many had levels that were still within laboratory standard normal ranges. 
The body's natural B12 receptor pathway can only absorb about 3 mcg at a time (about 10 mcg per day). The cells in the body can only use about 5 mcg per day and more than that is not necessary for normal biochemical processes.[12-14]
Not too little, not too much... just enough. That's a benefit of IF combined with B12 which allows for dosing between 2.5 and 5mcg - the right amount for absorption through the B12 receptors and optimal bioavailability of B12. [12,13,15]
This has been tested in a pilot human study  and it works. "What you need is what you get" with Xeragenx's IF. No megadoses in the gastrointestinal tract or circulation that have the potential to be 'not so harmless'.
Many multivitamins, prenatal vitamins and dietary supplements marketed for brain health or energy contain B12, but in huge doses to compensate for poor absorption.
Xeragenx has concluded its ingredient is Generally Recognized as Safe (GRAS) for other food uses, as determined by an independent panel of nationally recognized toxicologists and experts in food safety, following an evaluation of the safety of IF and related information, including production methods and purity criteria.
Many PNVs need a prescription because they contain 1mg of folic acid and may obscure underlying B12 deficiency and therefore require doctor supervision. Most PNV formulations contain B12 ...but without IF.
Adding IF to these formulations could allow lower B12 dosage to be given because of the high absorption through the body's receptor pathway.