Hi TDK--I'm with the others--that salesperson was clueless. Lift plates serve at least three purposes, only one of which is also adddressed by wider skis.
Lift plates make it easier to tip and hold your skis on edge, regardless of how wide or narrow they are. When you press down on your edge, the force that applies to your edge tends to fight your edging effort--it tries to flatten the ski, to reduce its edge angle. A lift plate reduces this torque, making it easier to keep your ski sufficiently edged. Indeed, the wider the ski, the more difficult it is to hold it on edge, and the more the lift plate helps.
Lift plates also affect the vibration and damping, as well as the stiffness, of the skis. Different plates have different effects, and more damping is not necessarily always what you want. But the width of the ski does not change this effect either!
Finally, lift plates can help minimize "boot-out." Boot-out is what happens when you tip your skis so far over that the boots hit the snow, releasing the edge just when you probably need it the most! Lifting the boot farther off the ski obviously allows you to tip the ski farther before the boot hits the ground. Since boot-out is not such an issue with wider skis, this benefit of lifts is lost on fats and mid-fats.
And lifts have downsides too. They are heavy. They can stiffen the ski under the lift. They can make the ski "slower" rolling from edge to edge. So you may well NOT need lifts on your new skis. But personally, I like a fair amount of lift on my skis, of any type. I find that the upside far outweighs the downside, for most skiing and conditions. I particularly dislike the feel of wide skis on hard snow without lift plates. I can really feel the increased effort in my ankles required to hold the wider skis on edge.