I fail to see the need or desire for the MapCreate 6 software, but I haven't played with it much yet.
From my reading and punching around on my 600c, the MapCreate will be able to increase the resolution and amount of the ground info, especially if you want to double over as a detailed roadmap guide. You don't get the street level resolution (like on-line maps) by default. MapCreate can also be used to plan out a route and then upload it to the GPS unit; saves batteries.
Now if I could only get the software to fully install. . . does not seem to like Win98/VPC6/MacOS10.4 It hangs on finding a file I know is where the installer looks for it (so it sez), and even when I navigate to the file for it. But as you say, it is not that important to have, unless you fly an amphib/floatplane and need the marine database loaded.