Blue Ray is not just competing against DVDs. It's competing against networks and hard drives.
You start with an $80 up front cost for a Blu-Ray writer, but then have to buy the discs, which have come down to $1 per disc, so $.040 per gigabyte.
Meanwhile. you can get a 1.5 TB external hard drive for $70, which winds up being $.051 per gigabyte.
So if you look at these costs, you'll have to use 290 Blu Ray discs before Blu Ray is cheaper storage than external hard drives. Yes, if you are just putting movies on those Blu Ray discs, you will be able to able to play those in a FEW more devices, but many DVD/Blu Ray/Consoles/etc. can read from an external hard drive. But access to a few extra devices comes at the cost of not being able to reuse that storage, where you certainly can with an external hard drive.
Also, if you are needing high amounts of storage, you are almost certainly doing video, and will wind up using a lot more than 25 GB. An external is far more compact (and easier to deal with) than 60 Blu Ray discs.
Sure, there are cases when you want to transfer smaller amounts of data, but that's what flash drives are for.