The "single biome planet" thing is a storytelling convenience, not an exobiological model.

It's like the "planet of the hats" where everyone wears specific hats: there to clue the viewer in rather than actually address timey-Earth millinery or Mirror Universe goatees.

From what we see here in the Solar system, single-environment planets look to be the norm - except they're wildly out of spec to harbor carbon biology.

Many of the conditions for carbon biology require liquid water, which has a fairly narrow temperature range and wildly different qualities as temps approach the extremes of that range.

Taking that fact in hand, planets with carbon life *should* have varying biomes simply because the temp range is so narrow but fairly variable within that range, imho. Even eliminating the effect of atmospheric gasses helping to balance temps out, the simple curvature of Earth forces temperature variances in the planet's water and that effect should hold true regardless of the planet in question such that you'd get varying biomes at varying latitudes. IMHO. Somebody check my logic, please.

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Pretty much zero. At the very least because there will still be a range of temperatures from the equator to the poles.

This assumes the entire thing being frozen solid or burning hot is not habitable. But even then, you'll likely have some variety.

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