The Res-Q-Me is a small keychain size tool that combines a protected cutting blade and a spring loaded punch.
The blade is designed to safely cut seat belt webbing and the punch will break tempered side window glass. They work because the one I carry on my turnout coat has been used a number of times in training and actual MVAs. Res-Q-Me is readily available for under $10.00.
Another alternative is the protected blade cutter often used as a letter opener. Very cheap, I have even seen the as promotional freebees.
The one problem with the rescue hammer is the inability to swing them by a confined or injured survivor. No matter which you choose, keep it easily available. This is no time to rummage around in the glove box.
While I always carry a blade (or two), I've never seen an MVA where a combination of EMT shears and a shielded blade tool necessitated using a knife. Use a knife if that's all you have, but there is potential danger from a struggling entrapped victum.
And NO Jim, we don't need another government regulation, although the EU approch is to control everything. (" Everything is either required or prohibited.")