Have you looked at your discs and had a feel at them yourself?

I'd suspect that the dealer is playing safe and/or looking for work. I've certainly been through my share of brake discs but the usage your describing shouldn't require replacement that soon. Surface rust will wear off and if you've worn grooves into the discs then some of the rust may still have been present when the dealer inspected it.

As for the dimpled and grooved discs, I'd say it's more of a cosmetic thing. I've certainly never noticed a tangible difference in terms of feel or performance. The only difference I'd say is that you can hear the ventilated discs when you brake as the pads passing over the dimples generates a faint sound of rushing wind. It's not loud though so I wouldn't let it put you off if you fancy going for them.