This is the third time I'm composing this reply... internet problems
The noise, size and obtrusiveness, in most situations, is more photographer-psychological, I agree.
Here's where post Leica M3 view finders have advantages over other systems:
1. the vf shows more than what's in the film frame. Therefore you can see what is coming into frame, whether by them moving, or by you re-framing. Once the eye is used to this it becomes very fluid and fast. SLR vf by contrast will usually show less than what is in the frame.
2. if the vf is bright and large you see better: in low light, regardless of speed of lens, or filters used.
Psychologically I find myself more 'inside' the camera vf when shooting metered SLRs, while I'm equally 'outside' when using a RF - more aware of the externals of the camera and scene. This is probably habit more than anything else.
And actually, shooting with a vintage camera may even be an anti-stealth advantage these days, as people think it's cute or interesting or cool, better than those black plastic monstrosities pointed at them.