You are right, it is a little extra money, but...
What if I don´t need such disk room right now? The benefits? A super silent iMac. Given all those threads about noisy disks, I´d even pay for a silence warranty.
I have an i7 with 256Gb SSD + 1Tb HD and to be honest, sometimes I think the HD was a waste of money. I have loaded all my files from my trusty G5 and still have over 120Gb free, I've not even touched my HD yet.
On the subject of noise, my HD is pretty quiet (just a faint fan-like noise) but as I'm not using it right now, I've 'ejected' the HD and it no longer makes even a the smallest sound. OK, so I don't use it but at least the option is there in the future and in the meantime, my iMac runs pretty silent.