You can't justify a free burning program that is easy to use and fits on a floppy over Nero which is easily ten times the size for the burning core of the program. The commonly used features are all on par and for the rest; they require the 10 separate modules Nero installs.
Now you could just say but it doesn't support burning audio cds, though last I checked that was broken from late 6.xx series to late 7.xx series anyways. (the broken mp3 conversion dll)
The lighter footprint allows it to run smoothly on older machines, so it typically decides not to freeze when the buffer runs out, unlike another program.
Imgburn costs about $79.99 less then Nero as well. In a world of rampaging pirates that doesn't mean a lot, but if free works better then pay, why bother jumping hoops?