A lot of folks want to use D cell batteries or other small cell batteries to get around patent issues (See Who Killed the Electric Car? if you're wondering what happened to NiMH EV batteries) or manufacturing problems. After all, laptop batteries and RC toy batteries are readily available!
So why aren't they used more, you might be asking?
The jiggle factor: Over time, things will work themselves loose. You're going to be securely attaching dozens of little batteries together, first in parallel clumps and then in series, with excellent fuses in between the groups. Every time you make a connection, that's an opportunity for bad things to happen, ranging from killing your expensive battery pack to killing yourself. A thousand separate opportunities translates into a lot of expensive and time-consuming battery babysitting.
Balancing: keeping tabs on a thousand or more individual cells requires serious and expensive battery management. As far as I can tell, there is no ready-made product out there that does this, no matter what they advertise.
Now that I've said that, though, there are serious EV builders who are using this approach. The Tzero and the Tesla come to mind.
Bottom line: If you're not a tinkerer, don't use D cell batteries in your EV project. If you're on a tight budget, don't use D cell batteries in your EV project. If you're allergic to setbacks, don't use D cell batteries in your EV.