1. Firefox remains the only mainstream browser to support OCSP Must Staple.
2. OCSP Must Staple does not cover all threat models: if an attacker gains the ability to temporarily issue certificates for the victim's domain (rather than obtaining the private key of an existing certificate), they can request a certificate without the OCSP Must Staple extension. A more effective method would be something like the Expect-Staple header[1] (in enforce mode).
3. It allows the ecosystem to move significantly faster. In a world where all certificates expire after 3 months, phasing out insecure hash algorithms (in certificates) would no longer take many years.
4. It encourages regular key rotation (even if it's not enforced)
[1]: https://scotthelme.co.uk/designing-a-new-security-header-exp...