Employability: Basically the title but I'll elaborate. Considering IB, Consulting and Tech (esp Amazon) are generally the more traditional areas for MBA students to target for recruiting, I've seen that you can get the same opportunities from lower tiered schools as well. For an instance, I've seen tons of MBAs from class of 2018 (even though placements stats aren't published yet) from Ross, Darden & Duke getting MBB, IB (both BBs and EBs) and tech offers. My main source was LinkedIn. You can get the same from M7 and maybe the numbers are a little higher than these T15 schools but, and I've talked to alumni and done my research, if you're well prepared, you have about 40-60% of chance of getting offers from any of these at T15 schools and the pay packages are about the same. Now my question is why do people stress more on M7 even though no one cares where you got an MBA 2 years out? There are some areas such as PE and IM where M7 outperform other schools but even at M7 as a qualified candidate, its an uphill risky battle and you don't have the absolute assurance that you'll get into those highly coveted areas. According to my research, half of them (even though well qualified) give up before recruiting begins because the process is just too daunting and risky.
General perception: I understand that brand is absolutely important for quant/engineering/econ and research programs such as PhDs and people are generally more impressed if you tell them you went to Columbia or Chicago for engineering or PhD than if you tell them you went there for an MBA. Because I believe MBA is not an impressive degree in itself and you don't need highly qualified professors to teach you corporate finance, fixed income, strategy or marketing. In fact, some of them have "professors" who are actively employed and come from professional fields to teach there as part time.
So the only point of going to M7 is better odds at those traditional career paths? I could be totally wrong here and I'm open to any input.