my understanding is that with modern materials, if a manufacturer is going to make a revolver in the .38/.357 chambering, assuming the materials are strong enough, it just makes sense to give it the extra chamber length to handle .357 mag.
The reason they say not to feed a .38 +P chambered gun +Ps all the time, or even to feed a .357 mag .357's all the time, is that it just is unnecessary stress on the gun.
I just bought a Smith 642 which is +P max, and the reason I got it over a .357 is that I've owned the 340PD before, but it's too light to shoot .357 comfortably from. I sold it not thinking that I may need a pocket gun. Now that I needed a pocket gun, I just figured for less money I could get a .38 special +P. It is only rated .38 special +P because of the materials, Aluminum and steel, whereas the 340PD uses scandium mixed with the aluminum to achieve a strong alloy capable of handling .357 magnum. You just pay extra for that.
There really is no reason to get JUST a .38 special if you could get the same gun at the same price for .357 mag. That's why you don't see that many guns chambered ONLY for .38 special anymore, because getting back to the first statement - with modern materials, it's easy to make a gun capable of handling .357 magnum.
The only exception is with concealed carry guns like my 642. It's lighter with the aluminum materials than a steel .357 mag. However, even with the lighter gun you can still get a .357 version as I mentioned - if you want to pay considerably more.
Also, of some of the guns still made only in 38, some are classics worth owning despite not being chambered for .357.
I'm going in circles.... I think I made the point.