Partitions give you options, period.
Want to experiment with a new OS, have a spare partition waiting, sure, go ahead.
Want to try a dual boot so you can take your time learning a new OS without screwing up the existing setup, partition already established makes that easy.
Part of your drive getting pretty fragmented, move the stuff you can to an empty partition and watch that defrag speed through whats left.
Granted the need to split big drives because window's does not recognise over XXX bytes is gone.
But all the other reasons for partitions still exist if you think about it.
In short, 2 minutes on the drive install can save you hours later.
Ohhh and last and best reason, if you don't partition your drive, your OS will be in C:/windows.
Right where every virus and malware knows where to look for it.
But if you have a partitioned drive, you can put your main OS on D:/Windows. I don't know how many times I've saved having to restart from scratch because I didn't catch the bug. But it is enough to be worth taking the time to make a partition or 2.