Your other question seems to suggest that a CA writes the CP, but this isn't the case.
A CP is written by the authority responsible for the PKI as a whole - usually called the PKI Management Authority (PMA). It is the PMA that owns the trust relationships between the entities within the PKI and it is the PMA which therefore does everything it can to ensure that trust isn't lost. As part of this it sets down rules (policies) which any Certification Authority has to follow if it is to be part of the PKI.
Once the PMA has written the CP, any Certification Authorities that hopes to issue certificates on behalf of that PMA must design the CA in line with the CP. Once that's done it must write a Certification Practice Statement that demonstrates how it is following the CP. Note that a well written CP will have audit policies that allow CAs to be checked for amongst other things, CP compliance.
A CPS without a CP is pointless. Without a CP the content of the CPS could be absolutely anything and it wouldn't be breaking any policies.
For example, section 6.2 covers the protection of private keys. Without a CP your CPS could say something to the effect of:
the Root CA private key is held on a USB stick stored in the stationary cupboard
The statement is irresponsible; but it doesn't break any laws, or policies.
However, if your management deem the above unacceptable and that the private key should be stored in a FIPS-140-2 Level 3 Hardware Security Module, then that what should be in section 6.2 of your CP. Once that's there, the previous CPS statement breaks policy and isn't acceptable. Attempting to submit a CPS with the above statement would result in rejection by the PMA and consequently the CA wouldn't be allowed to join the PKI.
It may be tempting to only write a CPS and submit it for review by either peers or management.
If your CPS is returned without any comments and accepted, you have to ask yourself whether the reviewers know their subject, or whether you've over engineered your CA thereby wasting time and money.
If your CPS is returned with comments (maybe suggesting how the private key should be stored amongst other things?) then these comments are the beginning of a CP. However, instead of many iterations of this review process and maybe heated discussions/arguments/firings, it would be much simpler if a senior authority (the PMA?) writes down the rules of running the PKI which all CAs must follow. In the long run, a CP will make the process of writing a CPS much simpler - after all, all the subjects need to be discussed at some point, so do it before designing your CA.
Finally, remember that there is only one CP covering all the authorities (CAs, RAs, VAs) in the PKI. Each CA (and possibly RA and VA if they're separate entities to the CA) writes a CPS to state how they're following policies.
As a minimum, Acme Inc will have a single Acme CP (written by the PMA), followed by the Acme Root CA CPS (written by the operators of the Root CA) and the Acme Issuing CA CPS (written by the Issuing CA operators).