Primal Strike

The original Primal Strike implementation is to make Enhancement Leveling more viable. Leveling as enhancement, a Shaman is still relegated to hurling lightning bolts like a middle-aged LARPer. Stormstrike is designed to replace Primal Strike at higher levels (once it is obtained), and thusly, they share a cooldown.

If PS is going to be replaced by SS whats the point? might as well just have SS available earlier

It's not like this is a new development. There are many talented abilities that completely replace baseline abilities. Some examples: Mangle replaces Claw, Scourge Strike replaces Obliterate, Heart Strike replaces Blood Strike, Hemorrhage replaces Sinister Strike, etc.

Claw is another example of an ability where characters of a different spec have little use for it (much like Resto and Elem Shamans will have little use for this ability). In fact, now that they are moving Mangle to level 10 (for Feral Druids), and you can't Shapeshift before level 10, Claw will become obsolete before you even learn it!

Edit: I stand corrected. It appears that you get Cat Form and Claw at level 8 in Cata. So aspiring Feral Druids will rely on Claw for a whopping 2 levels before replacing it with Mangle :)

Primal Strike's damage is dependent on weapon damage, but not in a multiplicative manner. It adds additional damage to whatever your weapon damage is. Stormstrike deals 225% weapon damage, and it adds a 25% crit buff on select abilities. This indicates that Stormstrike will replace Primal Strike as soon as you get it, since by the time you finish all your skills/rotation, the cooldown on Stormstrike will be over and ready for reuse. Keeping Primal Strike in your rotation will actually cause a dps decrease.