It's not necessary, helps for that early stage when you're getting ready for the full on hps veg or straight to bloom. It helps me sometimes with the transition so I have more control.
Make sure u dont do what i did, I left them in the cups for 3 weeks. They were root bound for almost 2 weeks. Growing pretty much stopped. Fixed the problem now though. ;P
The reason I like to get to a bigger pot fast, is watering. More soil, more moisture. I travel and need a good buffer zone; and a larger pot helps that.