Is there a good reason why they should?

Sony's trend is to make consoles with the latest format so i wouldnt be surprised if the next PS4 console used Holo disc. This may result with Xbox720 to use blu-ray and then devs would complain that the PS4 is too hard to make games on Holo disc and that blu-ray disc are much easier. =P

A better question is... is it feasible at all to have holo discs by the next gen, without making game discs and consoles cost a lot more than now?

what game will hold 500 gbs? 50gb disc can hold plenty and if it comes to it 2 blu-ray discs will be used, aren't they also onto making a quad layered 100gb blu-ray disc

That is also a very good question. With developers already willing and able to shoot themselves in the foot with big budgets in this gen, it doesn't look like a good idea to give them much more hardware power in the next generation.

In the next generation people will still be gaming with 1080p (or less) TVs, so cutscenes won't take up more disc space than in current PS3 games. Other than that, what needs more disc space so desperately? Textures could be bigger, but I don't think there will be a big graphical leap in the next gen, since development costs are already way too high.

Isn't there already a thread about this?

 Not that they will need 500GB discs for this, possibly more like 100GB discs,  but they could do what alot of Sony developers are doing, and that is put the same information on the disc multiple times to decrease loading times.  That would really be effective too given that the disc drive speed will also increase to likely 8x-12x instead of the 2x that the PS3 blu-ray drive has.   Plus Lossless audio will become a standard and that takes up alot of space as well.

Those things look more like using space "because we have it" rather than a real necessity since future BD drives will seek and read  faster (which reduces or eliminates the need for duplicating data).

I'm pretty sure some developers would use all the space if holo discs were the standard, the question is whether there's any substantial advantage to it.

No, they won't and neither will anyone else. This is simply another type venom from fanboys who want the BluRay format to die in its infancy. Holo technology is young and unstable and not to mention costly yet so its quite a while till we'll see it implemented in anything resembling home entertainment (or indeed, entertainment at all).