Well traditionally, the bija mantras can only be chanted by those who have upanayanam (sacred thread) and are Brahmins (by qualities). This is a verdict of all the traditional schools of Vedanta. If one who does not understand the power of these bija mantras and chants them improperly (because they have to be chanted in proper meter and tone) can do more harm than benefit. It is because these bija mantras directly invoke the diety, so one has to be very pure and chant them in the proper way. It is so sad, you see people everywhere showcasing mantras like the Gayatri even in the most dirtiest places.That is why the bijas mantras (like Gayatri) are passed from father to son, or guru to student often secretly. However the Lord's Name, has not such rules. Everyone is qualified to call upon the Lord's Name. That is why Lord Chaitanya Himself has said:
namanam akari bahuda nija sarvashaktis tatrarpita, niyamita smarane na kalah
"Oh Lord, you have manifested many names, into which you have put all your energies and power. Furthermore there are no rules nor times regarding the remembering of these Names"
That's what I thought. I for one would not chant a bija mantra and expect any benefit. There are bhajans that use them, but in that case, I think they are just lyrics in a song and have no meaning. The gāyatrī mantra is another one that is contentious. I see so many writings that say anyone can chant it nowadays, as well as the mahāmṛtyuñjaya mantra.
I myself chant them, not as japa, but during prayers, and I haven't experienced anything negative. Have I reaped any benefits from them? I don't know, maybe not. Maybe in that case they are just prayers and words in Sanskrit, instead of English or Hindi or Italian, or some other language used to talk to the gods, but I don't know. One thing I do know, or rather,
I feel, is that if they are used as prayers, they should be chanted or spoken with proper intention.
I'll freely admit that I have been one of those people who felt that almost anything is OK, but I'm kind of changing that thinking. Even the
Saguna Mantra for Japa page says not to use one of Kālīmā's mantras:
"OM Sri Maha Kalikayai Namah" (Prostrations to Mother Kali)
Kali is that divine aspect which is responsible for the destruction and eradication of negative qualities in this world. She is the transformative power of Divinity , which dissolves the individual into cosmic union. Maha Kali is one of the most fearsome of all the expressions of the Divinity. Because of the intensity of her purgative nature, very few people are initiated to this Mantra.
The bold is mine, and I'll bet people do use it anyway.