Key derivation after ECDH: why h(q ‖ pk1 ‖ pk2)?
I'm not sure whether it's the only reason, but adding pk1 and pk2 gives you more entropy, hence a higher security level. Like the docs say, the number of points on the curve is $\approx 2^{252}$, so for a 256-bit hash function $h$ you have $\#\{ h(q) \}_q = 2^{252}$ instead of $2^{256}$.