As to clipping a meter on top of the camera... About the only positive side of that I think is that it's a handy place to keep the meter, but has disadvantages too. Pentax used to make a meter that clipped on top of the pentaprism of the H3v model. I had one of those, and later when they made a meterless Spotmatic they revived the idea. These coupled to the shutter speed dial, and Leica has offered a clip-on for M bodies that does too.
If the meter couples to the camera, that's an added convenience, but it's still significanly less sleek and convenient than having the meter built into the camera body. If it's not coupled, as with the Voigtlander and Gossen meters, then the camera is simply a place to park the meter.
When you want to make a meter reading, you then must handle the whole camera and point it about.
If the meter is automatically setting the camera, as with a built-in, then this might make some sense, but I think otherwise it's smoother and easier to just waft the separate meter around to check the light, make note of readings for brighter and dimmer areas, and put it away until the light is different.
I have to admit to liking hand-held meters. They’re discreet. They often allow incident readings of the ambient light falling on the area. You can easily check what the light is like in different directions and use these readings to change camera settings as you move about without further reference to the meter. In this, I think the handheld meter helps the photographer learn about light and exposure.
Yeah, you can do this with a camera-mounted meter too, except for incident mode, which I prefer. But it’s more awkward. And speaking of awkward, I’m amazed by those using a metered camera just as a meter while shooting with a different camera. Desperation measure! Makes some kind of sense as a stopgap while shopping for a separate meter maybe, but I suspect folks just continue that way. Separate meters aren’t all that expensive...
BTW, I also like Auto and Program modes for dynamic situations with quickly changing subject lighting and with film having high exposure latitude. OTOH, being mentally prepped with hand-held meter readings of the different lighting anticipated, then making manual eposure changes on the camera, would likely give better results... but this takes more planning and discipline!
