Interesting question, NivipikLou. Welcome to EpicSki! (What is the origin of that name? Sounds vaguely Inuit...)
Many high-end skis now come with plates of some sort, as do many bindings. But so do some very low-end skis. Elan's great little PSX-short learning skis have a specially designed plate that makes them perform the way they do. They're excellent skis, but they are hardly high-end.
A few years ago, plates were mostly added on by the skier. They were expensive, specialized, and not universally liked. But you could expect that skis with plates on them probably belonged to a fairly serious skier, and were probably higher performance skis. Not so today, as so many skis have plates.
What really is your question? Are you considering buying a pair of skis with plates, trying to decide if they're good skis? If so, please give us some more information about both the skis and your own needs and ability. The more specific you can be, the more useful the answers you can expect. There's a lot of knowledge here at EpicSki--don't be afraid to tap into it with specific questions!