Easily fast attacks.
Heavy hitting but slower attacks can be good for Micro on a ranger, but generally they're not as good.
Miss one Heavy hitting attack, you miss 50 damage and have to wait another 3 seconds before your next attack to make it up.
If you attack twice as fast but hit half as hard, you'd only lose 25 damage and made that up in 1.5 seconds.
In addition, it works very well with passive abilities.
25% chance to stun for 3 seconds?
On average you'll have to wait 12 seconds for that with the 3 second attack, whilst with the 1.5 second you get it around about every 6 seconds or so.
5 HP lifesteal on attack? You'll get 20 HP in 12 seconds with the heavy attacks, 40 with the fast.
Of course, percentage lifesteal, and percentage critical hits and this are largely immune to such because the damage scales with the damage you deal.
This is largely speaking from personal experience too. Every time I play an RPG, RTS with heroes or MOBA, my friend always picks a speed oriented character. I'll hit harder than him, but I'm generally permastunned by his abilities, and he just dishes out DPS like there's no tomorrow.