If the only difference between preparing a five-minute song and a 15-minute song is the time it takes to do it, then an excuse made for that reason is a lazy one. I ain't sayin' it's not justified, and I ain't sayin' it's not reasonable, I'm just saying it's lazy.
Now the first part, the software limitations, that's understandable. I wish Harmonix would invest in software that allowed an audition mode to function past 10 minutes. That really doesn't seem out of line to me.