Simple answer:

Waxable classic skis -- If you're interested in racing and learning the technical aspects of classic skiing.

Skin classic skis -- If you're interested in fitness skiing in groomed tracks, and learning good technique. Skins tend to work better in warmer conditions.

Waxless skis -- If you just want to get out a couple of times a Winter, or classic with no maintenance bases.

Additionally:

Zero classic skis -- Work really well around 32 degree Fahrenheit temps.

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I tend to classic ski during the first half of the Winter when conditions are colder and softer, and skate ski during the second half of the Winter into Spring when conditions get firmer. Plus classic skiing requires A LOT less energy to maintain your glide than skating. I have to build a good fitness base classic skiing before I break out the skate skis.

Just understand there are definitely "classic" days and there are "skate" days. That's why it's good to have both.

I have THREE pair of skis: skate, classic skin, and classic waxless touring. Those seem to cover all my bases depending on conditions and location.