By chanting during Japa, we perform service to the Supreme Lord and by hearing it we do worship to Him. As we hear more and more, our chanting become increasingly perfect. As our chanting becomes more perfect, our worship to The Lord becomes successful and we get more opportunities to serve. Thus our life becomes sublime.
By this twin process, whatever deficiencies we had at the beginning, is gradually washed away. Thus chanting is a foolproof process and can be practised by anyone, however fallen he may be. This is the opinion of our previous acharyas. I therefore do not contribute to the opinion that offensive chanting is worse than no chanting.
A beginner will always do offensive chanting because his mind is not yet ready, but the cure for this malady lies in chanting more and more. While we should avoid deliberate offenses during chanting, no one should be barred to chant on the ground that his chanting is not offenseless. In Caitanya Caritamrta, Mahaprabhu is glorifying the chanting by saying:
iha haite sarva-siddhi haibe tomara sarva Kshan bala ithe bidhi nahi ar
The meaning of this verse is - By chanting of the Holy name, all your aspirations will be satisfied and this chanting can be performed under all circumstances, there are no other rules and regulations.
As the age of kali is very fallen, Mahaprabhu (the Lord Himself ) has made this process very simple to give everyone a chance to go back to Godhead.