Answer

Is an f/1.2 lens worth it over f/1.8 for portraits?

For most people, no. The blur difference at portrait distances is modest, the depth of field at f/1.2 is thin enough that one eye can miss focus, and the lens is typically two to four times the price and roughly twice the weight.

What the extra stop buys

About 1.3 stops more light, useful in dark venues
Slightly stronger background blur at the same framing
Often a distinctive rendering that some photographers specifically want

What it costs

Focus accuracy. At a tight headshot, depth of field at f/1.2 can be under two centimetres. Subject sway alone can ruin a frame.
Money. f/1.2 primes commonly cost several times an f/1.8 of the same focal length.
Weight. Usually double, sometimes more, which matters across a full shoot.
Corner performance. Many fast primes are softer and heavily vignetted wide open, so people stop down anyway.

What working photographers actually do

Published shoot data and photographer write-ups consistently show portrait work clustering at f/1.8 to f/2.8, even when a faster lens is mounted — enough separation, comfortable margin for focus.

When f/1.2 makes sense

Available-light wedding and event work, environmental portraits at longer distances where depth of field is more forgiving, and photographers whose look depends on a specific lens's rendering.

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