There are many good reasons for wanting the keys to be based on the TPM generator. As the source for the kernel random number generator itself says, some systems lack good randomness at startup, and systems should preserve and reload the pool across shutdown and startup. There are use cases for trusted keys which need to generate keys before such scripts have run. Also, in some use cases, we need to show that trusted keys are FIPS compliant, which is possible with TPM generated keys.
If you are able to call tpm_get_random(), the driver has already registered TPN as hwrng. With this solution you fail to follow the principle of defense in depth. If the TPM random number generator is compromissed (has a bug) using the entropy pool will decrease the collateral damage.
I.e. you make everything depend on single point of failure instead of multiple (e.g. rdrand, TPM, whatnot).
/Jarkko