Monitor RGB disables / turns off display color management in Photoshop. It cripples the added accuracy that color management gives. It therefore, miraculously, is suddenly consistent with all the other applications that have no color management, of which there are many. That's why this incredibly bad piece of advice is all over the internet and is impossible to put to the merciful death that it deserves.
Listen, this isn't rocket science, or quantum physics. You convert to sRGB because most monitors conform pretty closely to it, and that includes "big iMacs" by the way. So it's foolprof, and will display roughly correctly in any scenario, with or without proper color management. Believe me, foolproof is needed. Adobe RGB is in any case wasted on a standard, even "big iMac" monitor.
The exception is wide gamut monitors. But those who have wide gamut monitors are usually educated enough to know what they're looking at, and can take the necessary precautions.
With an sRGB file, those who do know the basics of color management, and use color managed browsers, get to see it properly. But it's not those you need to worry about. You post sRGB for all the others.