Importing another module takes non-zero time and uses non-zero memory, and let's face it: python is not exactly a fast language. Personally I'd appreciate a library author that takes steps to avoid a module load when that module is only used (for example) in some uncommonly-taken code paths.

In some (many?) cases it's probably premature optimization, but it doesn't hurt, so I don't see why anyone would get up in arms over it.