The Zone System makes sense if you develop your B&W negatives one by one, separately, and you print yourself. When using roll film in general, and a colour film in particular, you adopt the same development for all exposures of the roll.

Digital clips the highlights abruptly. Slide film burn highlights much more gradually. Negative colour film is even more forgiving. B&W negative is extremely forgiving. A spot meter is especially useful to avoid burning highlights, while retaining details in the shadows, when using slides. Otherwise it is not indispensable.

With colour negatives your S90 can serve you as a decent reflected light lightmeter. A spot exposure meter or an incident light exposure meter are useful additions to your gear set in any case.

I shot a couple of rolls of colour negatives recently (outdoors, sunny) metering "by sight" with no problems.

Fabrizio