This TGV route is not just from the airport to DLP - its a long distance train and just happens to call at Marne la Vallee (DLP) as its first stop after leaving the airport.It definitely can vary with pricing and timing. For us the price will be lower on the TGV for 2 adults and 2 kids, especially if we get a Carte Enfant+ discount card for one child (because we'll have multiple train legs on our trip around France this makes $ sense). The timing works pretty well for us too, and we arrive in terminal 2 so its a <10 minute walk.
I'm sure for many people a shuttle seems more convenient and familiar, or works out to be cheaper or faster, and there's nothing wrong with that. For those unsure, its worth pricing both out and looking at time tables (on oui.scnf). Trains are cheapest when they first go on sale (3 months for regular TGV, 9 months for the Ouigo discounted line.) In fact the Ouigo times for our travel day in April are already sold out, so this is certainly a popular way to get to DLP.
Europe has a much better rail system than the US so taking public transit is always the starting point for our family, then we fill in with taxis/shuttles as needed. (In 5+ trips I can only think of a few times I've been in a car, its just not necessary.) In fact the TGV is one of the fastest trains in the world, when I visited in high school it was a highlight of the trip. I'm pretty excited they put a TGV route just to DLP. There's something very cool about traveling at 225 mph, especially for American kids who've never done that at home.