Only when there is a problem or special case would it be necessary to use this. This was put in to be used when some driver was hanging on a NE2000, or misidentifying some other device as its own. A correct device driver shouldn't probe a reserved region, unless another boot argument explicitly specifies that it do so. This implies that reserve will most often be used with some other boot argument. Hence if you specify a reserve region to protect a specific device, you must generally specify an explicit probe for that device.
Most drivers ignore the port registration table if they are given an explicit address.
As usual with boot-time specifiers there is an 11 parameter limit, thus you can only specify 5 reserved regions per reserve keyword. Multiple reserve specifiers will work if you have an unusually complicated request. The command lsmod will show you what modules are loaded, and rmmod will remove them. To actually generate the modules, you have to type make modules after you have finished building the kernel proper. Earlier kernels built them automatically, which wasn't fair to those compiling on 4MB 386sx-16 machines.