okay, this has been bothering me for a long time... why don't they ever use a pensieve? like in PoA, noone believed harry and hermione since they didn't have wormtail and w/e but why couldn't harry just pull the memory out an show everyone...or in court cases...like sirius could've shown wormtail becoming secret keeper and stuff...so like why do they only use a pensieve for useless things (well not really but you know what i mean) when it could save lives?

That's a really good question. Perhaps Pensieves are really, really rare, and therefore use of them is highly limited. And, of course, the Ministry is corrupt, so they probably wouldn't have accepted use of the Pensieve anyway. And, as demonstrated by Slughorn, memories can be tampered with or altered, which would cause one to question a Pensieve's credibility.

I think the first is most likely, that they are really rare and thus have limited use.

I think pensieve's may not be trusted for all things. As we saw with Slughorn, memory's can be tampered with. I don't think that they could always take a pensieve - or someone else's memories - completely seriously due to this fact. We also saw with Morfin and Professor Lockhart that there are memory charms and such that can remove a certain memory or implant another memory in someones head. So I'm assuming that if Harry/Hermione were to have shown their memories through a pensieve to the Minister or anyone else, he would have just assumed it was a memory planted in their heads by Sirius Black or something to that extent.

Good point but what if the person just imagined all that stuff and remembered it. It didn't necessarily have to be true. My friends have made stuff up and remembered it. It would still be a memory.

About Sirius showing his memories of Wormtail, Sirius didn't get a chance to defend himself because he was never given a trial. He was one of the people who were arrested by Barty Crouch Sr, without a trial. It said that in GoF.

Even if Sirius had shown them a memory, I don't think Crouch Sr. would have set him free. He would probably think that Sirius tampered with it to make himself look innocent. And then there would be the witnesses who supposedly saw Sirius murder Pettigrew.

What if a pensieve is like an invisibility cloak? Not like Harry's cloak, but like the ones Ron talks about, like the ones that lose their magic over time. What if the Pensieve will lose its ability over time as well? Just a thought. What you think?

Right when i got done reading GoF, i thought the exact same thing. They could save so many innocent lives, and put away a lot of bad people by using the pensive. I first realized this when i read about Harry's hearing about the Dementors. He could have just shown them what happened. Then the more i read through the books, the more i realized that a lot of the book's would be completely different had they used a pensive. Maybe there was a law that said that a pensive wasn't fool-proof. If you don't remember Prof. Slughorn changed around his memories for the pensive... But regardless, the books probably wouldn't be as interesting if they had won ever single case.