Let's say you had to swap disc from 1 to 2.  There is common data that has to be on all discs for the game to properly function.  If tha data of the main controlled charatcer is NOT on discs 2 and discs 3, how would you control the same main character?  You can't.  Now sometimes the same data is also repeated on a single blu-ray to help reduce seek time.  That depends on the programming and programmers.  But if you are controlling the main character throughout a three disc (DVD) game, you have to have three time the same data amongst three discs as opposed to it once on one DVD and possibly one Blu-ray.  Therefore, your total space amongst multiple DVD's are not used as effectively.

Look at the post I am quoting in the tree.

Stupid vgc -__-

"when it mentions side missions, it seems to allude to that if you wanted to complete some of them, you would need to swap disk more than twice. " 

i think what he means as there are constants on all the disks, as an example, the main characters textures and expressions must be saved on all disks because they'll be called upon from all disks.

another example would be an overworld map. to work in a video game (like final fantasy 7) the overworld map has to be compelete on all disks so each final fantasy 7 disk only had like 400mb of space (out of 700mb) to put actual content because the data had to be redundant across all the disks to ensure you could travel wherever you wanted at whatever point in the game

without being redundant across disks in this case you would either require no exploration to places you have previously been to (final fantasy 13) or disk swapping to access places that you had previously been to (mass effect 2).

Explain this to me, I don't see what you are saying...

You said:

"Swapping while trying to do side missions would severely interrupt the flow of the game. Come on, this is fun. How else can you twist it? "

I don't think he said anything about "severely interrupt."  You did, most likely to paint a false picture to argue against to make him look bad.  Why would you do something like that?

No, I wasn't... I was agreeing with him. They could of just made it a multi-layer blu-ray to add more content, as well as just adding a new DVD. 

No, my math is right, your knowledge on dual layer DVDs is off. A single layer is 4.7 GBs, a dual layer is 8.5 GBs. 

8.5 8.5 8.5= 25.5 

25x1=25 

You are taking something and trying to warp it in the worst way possible. All the multi-game discs I played I could travel all I want. I only had to swap for story segments. And lets look at it one more time: 

In fact, players will only need to swap discs twice at natural breaks between cases without interrupting the flow of the game

Swapping while trying to do side missions would severely interrupt the flow of the game. Come on, this is fun. How else can you twist it? 

Let's say you had to swap disc from 1 to 2.  There is common data that has to be on all discs for the game to properly function.  If tha data of the main controlled charatcer is NOT on discs 2 and discs 3, how would you control the same main character?  You can't.  Now sometimes the same data is also repeated on a single blu-ray to help reduce seek time.  That depends on the programming and programmers.  But if you are controlling the main character throughout a three disc (DVD) game, you have to have three time the same data amongst three discs as opposed to it once on one DVD and possibly one Blu-ray.  Therefore, your total space amongst multiple DVD's are not used as effectively.

I know you'll get the PS3 version anyways, but just to answer for other people that may of missed it. You will only have to swap discs twice, between cases, or so says the original article 

@Bold: Where do you get this? O.o 3 dual layer dvds equal 25.5 GBs. A little over that of a single layer blu-ray. If anything they cut content to allow only a single layer blu-ray. 

@ Second Part: Read my response to Skeeuk