Some painters do that...I really have no idea why. It's probably not cutting a corner though. The high quality exterior paints cost about the same as the high quality interior ones. Some paints like Grahams Satin Ceramic are labelled as interior/exterior. Exterior paints just have some properties to deal with vast temperature changes, humidity, mildew, etc...stuff that you don't really have to deal with inside. Latex paints take up to a month to fully cure and harden. Anytime you have a painted surface contacting another painted surface before it's cured, you may have problems with sticking and paint coming off. I try to leave them unlatched at least for a few days or close them lightly so that one painted area is not mashed against another. The more days you can give them the better. Exterior paint is not thicker as far as I can tell.

We had a similar decision to make except that the casements were to open into a screened porch and we were concerned that while on the porch, people might run into them. We have double hung windows with grills on the top half all throughtout our home but I didn't want my sink centered on a middle piece of wood between two casements. Was just something I didn't personally like for our kitchen. We then saw a similar window to what is in my most recent post which has sliders where the side casements usually are and a large picture window in the middle. WE LOVE IT because it gives us unobstructed view, still has the venting options, screens are on the outside and we felt that having the option of the grills on the side windows, helped it to look more traditional like the rest of the windows in our home....our window is one of the best choices we made - we just love it! I copied my link to my pictures below....you'll see it in the 4th pic down over the sink. Here is a link that might be useful: imlebby's kitchen

You know, you might benefit from cross posting in the building a house and decor forums. Lots of folks there who might be able to give you some great advise.

@BeverlyFLADeziner: I have an open concept floor plan. The kitchen is open to a combination living room and dining room. My color theme is black, white, and gold. All of my walls and mouldings are white, the kitchen cabinets and island is white, and the countertops are white with grey veining. Im going to paint my kitchen island black. I also plan on painting my doors black.