Dialog hacked version with no install, posted as found...

Two words, simplicity, and ease of use.

Renaming syntax is minute (traditional wildcard characters '?' and '*', '\l' for lower case, '\u' for upper case, '\i' for char\string insertion, '\-' for char deletion, and that's it!) and mnemonic (I'm so used to it that I didn't even need to look it up, but it's also available from MultiRename interface).

When more convoluted operations are needed, they can easily be broken down into consecutive steps (a simple but extremely common example: you have a series of files with names like 'file1', 'file2', ..., 'file9', 'file10', and so forth; first, I'd normalize the names of one digit files, and then do the mass renaming).

Undo functionality (via an extremely convenient auto-generated batch file, allowing for both documented changes as well as fine-tuning) is a definite plus when working with critical files.

Last, but not least, the availability of the program's GUI, only two clicks away from anywhere in the file system, making real easy to preview and adjust for desired outcomes.