Not maintaining the integrity of the rhythms of mantras is equivocal to not maintaining the correct accent of words in Mandarin chinese. I know it sounds extreme, and it's a way to convey how I feel about this. If you don't maintain the correct accent of words in chinese, no one will understand you. For example, if I changed the accent of banana, it could also mean elephant's foot. Yeah, no kidding. When you strip away the correct pronunciation, and the rhythms of mantras, you don't have a mantra anymore, you have a song, or some ordinary series of words strung together. The power of the sound is gone, and has been replaced by noise.
If you are serious about the practice of mantras, (I'm referring to Vedic mantras in particular), then you will value the structure of mantras. The word mantra can be broken into man, manas or mind, and tra, tool. Mantras are mind tools that were created and perfected through their power in sound. Sound has many levels of depth of understanding. A sound is more than just a sound, it is a vibration, and conveys an idea of an object; it is imbued with meaning. And sound also indicates the consciousness of the person, and conveys their subtle presence. This subtle presences is none other than turiya.
When you bastardize a mantra by changing the rhythms and melody, you are smashing the depth, and flattening it to the grossest level of manifestation. Please do not engage in such an insipid form of practice in front of me. And for you integral geeks out there, here's some KW to back up my fierceness on this topic:
Mantras are Zone #1 injunctions that can help with particular state training, and can induce ordinary and nonordinary peak experiences. "When you correctly follow those injunctions, you will have a series of phenomenological experiences." -KW, Integral Spirituality, Ch. 3 (My italics.) Yes, correctly, because otherwise you are just wasting your time.