Some have argued that it is better to get a 1.4 even if you aren't going to use 1.4 but because if you close the aperture down to f/2 on a 1.4 lens it is "supposed" to be better image quality and sharpness than f/2 on a 1.8 lens. Not sure if that is true or not though. I have the 1.4 and like the fact that the aperture is slightly larger for more shallow dof and lets in a bit more light, giving me more leeway so I could choose a lower ISO and have a less noisy image. I don't actually need a lens with and aperture as wide as f/1.2 and even if I did, I couldn't afford the fecker!.
As to STM, not sure I'd need it for my personal use "in most cases". For portraits I generally use shallow dof and the range of aperture range usually goes from f/1.4 to f/5.6. When shooting at f/1.4 my shutter can be as high as 1/1250th sec, it's not like I am going to have motion blur using a shutter speed that high. I could see the case for STM if you close the aperture down and are shooting scenes like 1/10th sec.