Quite a few threads discussing that it was the greatness of Gulzar's lyrics and RD's music more than Amanat's singing that made Amanat's rendition seem so good. I myself participated in some of these discussions and I reiterate what I said. Amanat was even better than Anup Ghoshal in my opinion. Anup Ghoshal's rendition had a lilting quality to it. Amanat's was just so soulful and so full of pathos, it gives me goosebumps and brings tears to my eyes every single time I listen to it. Oh yeah, as for Samir's rendition, does not even come close to Amanat's. Other than the childish voice, it just did not have the subtle nuances that Amanat's rendition did. It was more the atmosphere (all the lil champs singing for their parents) that did the trick there. Anand Raj Anand said it very well about Amanat's performance 'Ab tak mehfil mahaul ki ghulaam thi. Isne is ibaadat se gaaya hain, isne to mahaul hi badal diya'.
As for the contention that any decent singer can do that song justice; maybe. However, there are good performances and then there are great performances and Amanat's performance was GREAT in my opinion.
Here are the three performances for comparison.
Amanat
http://www.youtube.com/watch?v=gpZEWzhtPh0
Anup Ghoshal
http://www.youtube.com/watch?v=E_WV6HRq3SM