I am 52 now. I have been a lifeguard for most part of my life. I quited as a lifeguard and a dive rescue team leader in 2007 when I left Germany. When I started scuba diving about 30 years ago, I did it the old fashioned way, with a lot of swimming and snorkeling, so I think I know what I am talking about.
Are good swimming skills and stamina helpfull for scuba diving? - Yes.
Are good snorkeling skills helpfull for scuba diving? - Yes.
Are they necessary for scuba diving? - NO!
Is it necessary for scuba diving to feel comfortable in the water? - Yes.
Is some level of general physical fitness necessary for scuba diving? - Yes.
During my carreer as a diving instructor, I had several students who did very hard to pass the OW swim test, but they became very good divers.
And one of the most important points is to know your limits. If you are not a strong swimmer, may be its not a good idea to dive in rough conditions. Also, if you are not a good swimmer, may be its not a good idea to dive very far away from shore or boat. But if you know your limits and stay within them, where is the problem?
If someone wants to swim for improving his physicall fitness, why not? If he asks me for some advice for improving his swimming skills, why not?
But I don't see having my student divers swimming and snorkeling as a necessary part of my job as a diving instructor. I just do it because the standards of my agency force me to do it.
IMHO its a waste of time during a scuba diving class and its one of the remnants of the "good old time" when every scuba diver nearly had to be a combat swimmer.