The debug item is rather useful in creative mode. Fortunately, I had NEI, or I wouldn't have found it, as it isn't in the normal list.
Anyway, the infinite energy feature is very useful when designing machine setups. However, it would also be useful if it could also power furnaces/generators/anything else that burns things indefinitely as well, so I can design with a power limit but not a fuel limit.
For example, I often setup a bunch of stone engines from Buildcraft. It would be useful if I could just plonk down some debug items to fuel them, instead of sending them stacks of Coal Blocks.
Naturally, the debug item would remain available only through commands and NEI (and hopefully creative as well).