Google Voice and mobile phones.. am I missing something?

Google Voice and mobile phones.. am I missing something?
So everyone is a twitter about google voice and using the service for all sorts of things. Having it call you and call a contact to make a “free” call. I’m an AT&T wireless customer and do not have a home land line, I dont see how I can benefit from using google voice to make calls for me. Let me explain:
My AT&T plan gives me X minutes of talk time per month and “unlimited” talk time after a certian time and weekends. When I call a friend using google voice it calls me then calls them and we can talk. By doing so it’s eating up minutes on my phone since AT&T doesnt provide free incoming calls (unless something changed I don’t know about). Lets say I use the GV method and call a fellow AT&T customer, now I’m really screwing myself since we both have AT&T and we’re not using our unlimited mobile to mobile talk time.
The only benifit I see here is that if you’re calling someone from a land line and want your GV number to show up on caller ID then you’ll be able to do so for free.
Am I missing something? Let me know in the comments
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Google Voice

So everyone is a twitter about Google Voice and using the service for all sorts of things. Having it call you and call a contact to make a “free” call. I’m an AT&T wireless customer and do not have a home land line, I don’t see how I can benefit from using google voice to make calls for me. Let me explain:

My AT&T plan gives me X minutes of talk time per month and “unlimited” talk time after a certain time and weekends. When I call a friend using google voice it calls me then calls them and we can talk. By doing so it’s eating up minutes on my phone since AT&T doesn’t provide free incoming calls (unless something changed I don’t know about). Lets say I use the GV method and call a fellow AT&T customer, now I’m really screwing myself since we both have AT&T and we’re not using our unlimited mobile to mobile talk time.

The only benefit I see here is that if you’re calling someone from a land line and want your GV number to show up on caller ID then you’ll be able to do so for free. Outside of that your paying for hiding behind your google voice caller id?

Am I missing something? Let me know in the comments please.