I don't think need park hoppers, but they can be pretty nice to have. I like to think of DLR as Epcot: DL is Epcot's Future World, DCA is World Showcase, and DTD is just outside of International Gateway. The Esplande, the area between the two parks, is about the same length as the bridge that connects WS and FW. Going between DL to DCA is usually pretty easy (there are times when the tickets lines can be pretty lengthy, but if you avoid crossing at that point, it's usually less than 5 minutes). I like having park hoppers because it lets you treat the resort as one big park. If something is down, you can easily go back to it a different time/day. Same if you see that something has a really good wait time or if there is a show you want to see or somewhere you want to eat.
That being said, I've gone to DLR and spent the entire day in just one park. Different people tour differently and if you plan it out, you can do one-park-per-day without missing out. I've just gotten so used to having park hoppers, it's tough to imagine a four day trip without hopping!