What use is Snape in the rest of the plot?

[Jospehine]

No: HPFGUIDX 132997

My last post for the day- lest I over cram the message board with all 
of my babblings-*sigh*!

Right. Snape Snape Snape... so, it would appear that most of you 
think he's on the good side.

Considering where book 6 leaves us, it is suggested that Book 7 will 
consist of:

Harry's hunt for the Horcruxes
Harry's destruction of Voldemort (or failure to do so)

That is pretty much the essence of it, stripped down to its bare 
bones. No? 

The question I have then is this- if Snape is still on DD's side, 
what does Harry have to gain from his allegience? It's not in Snape's 
character to help Harry, so I can't see him going 'oh look Harry 
here's that Horcrux you were looking for!'. Other than postponing 
Voldemort's knowledge of Harry's hunt, what else can he offer to the 
OoTP? We already know that Voldemort is intent on mayhem and 
destruction. Tell us something new Snapey! I can't help but feel that 
his betrayal of Harrry/DD/ et al was the only way his character could 
have been written- otherwise he was in danger of being shadowed into 
obscurity with no further purpose to the cause.