From what I see, many server and client RSA certificates in the real deployments have the "key usage" mask containing both digitalSignature and keyEncipherment.
Isn't this a bad (that is, potentially insecure) practice, given that there's a general guideline to use separate keys for different purposes?
As an example of protocol that appears to freely switch between this two, take TLS 1.2. If the key exchange method, that was negotiated between the client and the server, is ECDHE_RSA, then the digital signature capability of the certificate will be exploited during the handshake; if, on the contrary, the method is plain-RSA, then the key encipherment capability will be used. And these two specific instances are probably still widely used - assuming this after checking which methods have been deprecated by the most popular browser Chrome and which have not.