The Depth of Field preview button- is it very necessary?

To understand the DOF preview button you have to understand what your lens is doing when you look through the viewfinder (VF). A lens is ALWAYS going to show you the widest aperture for any given focal length when you look through the VF.

In case of prime, if you have a 50mm f/1.8 then it is ALWAYS going to show you f/1.8 when you look through the VF. Even if you are shooting stopped down to f/4. In order to see what the camera will ACTUALLY capture when you PRESS the shutter you need the DOF-preview button. And thats why it goes dark: the diaphragm on the lens actually stops down to the aperture you've set (e.g. f/4) and this means less light is coming in. But you will see that the DOF of the composition increases when compared to looking through the VF without pressing the DOF-preview button.

In case of zoom with constant aperture you will have the same problem, it will be set to f/2.8 or f/4 or whatever the constant aperture setting is.

In case of zooms with variable aperture say 28-135mm f/3.5-5.6 you will still see the view from an f/3.5 at say 28mm even if you are taking the pic at f/8 and a view from f/5.6 at 135mm even if you are taking the pic at f/8. Again if you press the DOF preview button at any focal length and the image in the VF darkens then what you are looking through the VF is the widest aperture for that focal length even though when you press the shutter the lens will stop down to the aperture you set for the pic.

Is it really necessary?

Well that depends on if you really care for the DOF. If you do then its a good idea to preview the DOF before capturing it. But if you are on a DSLR you can always chimp: look at what you got and if you dont like it shoot again with an appropriate aperture.

A half pressed shutter, at least on every camera I've owned (six to date) does not give you depth of field, on many autofocus camers it will lock the exposure and focus point (on some its one or the other) but it will not give you DOF.

I use it from time-to-time, but if you never had one, you will probably never miss it. It's a nice feature on a high-end camera, though.