HBDeathshot, ZG enchants are not as readily available as Libram enchants, plenty of people don't raid at all, don't have the DKP to win an idol or have alts that don't raid.
In response to the original poster, for my hunter I use haste enchants on gloves, head and legs. The extra autoshots are a direct DPS increase but also additional chances to crit (which gives extra damage and procs Go For The Throat for 200-500 Nature damage) and to proc 'on hit' events such as Black Grasp of the Destroyer and Improved Aspect of the Hawk. It's also faster / extra caster interrupts in PvP. Basically it lets me get better mileage out of my existing RAP, crit chance and talent spec (at a cost of higher ammo usage).
Depending on your class, level, gear and talent spec there will be a breakpoint where you may get more out of rapidity than +8 Agility on head/legs. Ability usage will also come into this - if you are a rogue and use Gouge a lot then Rapidity will not buy you much, but if you're a Fury Warrior or Combat Rogue in a raid environment you may get more out of haste.
Looking at Agility versus Haste in isolation is useless however, particularly as there is no such thing as 1% haste anymore - it's 10 haste rating which scales with your level. Just to illustrate with an example, 10 haste rating at level 60 gives 1% haste but for a level 21 it gives 4% haste. Agility on the other hand gives the exact same amount of Attack Power at level 60 as at level 21.