Paolo, bear in mind that a plate reverb is simply a speeded-up 2-dimensional model of stereo reverb from a mono omnidirectiional source in a rectangular room. It's speeded up because the sound is travelling much shorter distances in the metal plate, compared to a real room, and so the early reflections and overall reverb build-up happen very much sooner. Hence it can be used very effectively wherever you want very fast wetting, or wherever you want to assert reverb build-up sooner rather than later.
Traditionally, plates have been used extensively with drums; it's always been the best way to get a smooth and dense reverb profile almost immediately onto percussive or other short sounds. That said, there's nothing to stop you using a plate on sustained sounds, if your ear tells you it works better than a slower build-up artificial reverb.
Note that a plate is inherently, like many if not most mono-to-stereo reverbs, spatially very 'single-minded'. Depending on how much of the plates you want in the mix, you may have to totally re-spatialise artificially after the plates.
And finally, don't forget that not all digital models of a physical plate reverb (usually the EMT-140) are created equal. And there's a huge selection on offer. Moreover, still-existing physical plate reverbs are themselves very variable beasts, and so even the most meticulous digital emulations of actual plates may sound somewhat different to one another. For those interested, here's one useful comparison review of some plate plugins, including comparisons with an actual EMT-140:-
What is the best plate reverb plugin?
