Perhaps I should clarify what I wrote earlier. First I said, "mostly, I find manual exposure faster and invariably more accurate". Obviously, it's only "invariably more accurate" if you have time to use it.
Second, I was referring to negative film: "if you're shooting negative". Slide film is different. You need more precision, and exposures are keyed to the highlights, not the shadows as they are with negative.
With negative film, exposure is very much a matter of opinion. Over-exposure changes the tonality (generally in a way that people prefer) and reduces sharpness while increasing grain size (with conventional B+W). Very slight underexposure will give higher sharpness, at the expense of tonality, but any more than slight underexposure will cause image quality to fall off a cliff. That's how negative film speeds are determined, after all: by the minimum you can give.
For a hand-held shot the loss of sharpness from over-exposure will normally be far less than the loss from camera shake and not using the optimum aperture. Normally you'll have plenty of time for metering if you're using a tripod.
With grain and sharpness, therefore, the penalties for even a stop of over-exposure are normally trivial, and 1/2 stop really is negligible. With 1/2 stop rests, you can never be further than 1/4 stop from optimum exposure, if you know what exposure you want and how to meter for it. There is absolutely no point in reducing this to 1/6 stop difference (achieved via 1/3 stop rests) with negative film: the variations in exposure are within experimental error, given any normal metering technique and through-lens metering. With auto-exposure, variations are well within experimental error: if you get the ideal exposure within 1/2 stop you'll be doing well.
In other words, shooting intermediate shutter speeds, when shooting negative [film] is presupposing more precision than exists or is needed in the system -- especially on auto exposure.
Finally, most (I believe, all) M-series Leicas will in any case give you intermediate speeds if you set the shutter speed dial between numbers.
At this point, the main distinguishing technical points of a ZI (disregarding "feel", noise, ergonomics, meter readouts and durability) are AE and battery dependency. The choice is then one of priorities.
Cheers,
R.