Yeah its a combination of two things:
1) For most of the day, in my area and at my office the speeds are 2G or low 3G at best. So no matter the FUP, the network is useless for everything aside from checking mails.
2) At night however, esp during the free hours, speeds greatly improve probably due to correspondingly lower no of users. And for me atleast, they used to be high enough I could easily download upwards of 10-15 GB every night during those 3 hours.
With calling largely unavailable as well on the SIMs I have, if I can't even use internet for most of the day then night unlimited was my only hope of proper data usage. I have 4 MBps BSNL broadband in my area as the only reliable wireline service, so I was really happy to have those high speeds and unlimited downloading ability at night. Even I would be willing to pay 100-200 Rs for unlimited night usage as the speeds are likely to be much higher and that would be great value for money.
I guess it would depend on the no of prime subscribers at the end of this month. IF their targets aren't met, won't be surprised if they offer discounts/other plans to lure more customers.