Are the original Starcraft 2 WoL and HotS DVDs still useful for installing the game?

Yes it will reduce the amount of data that you will be required to download.

From Blizzard Support Rep:

[the disk] contains a pretty up to date client [1.5.x (stated in another comment)] which will take you some way but as with all of our games, StarCraft II is constantly evolving and it will reach a point where downloading the client on a broadband Internet connection will be quicker and simpler than installing from a disc and then patching up with the latest data.

By how much though is a very difficult to answer question without actual testing of the build on the disk as well as the current production release available to download. It would depend on many things such as how the patching system works and how much data has been changed since the DVD version.

As a guess I will say that it may save up to 4gb.

A possible solution for you may be to download and install the game somewhere else, perhaps at work or school (if they let you) or a net cafe, and then copy the install files onto an external harddrive and then bring it home and copy it onto your home computer. This will likely mean you will not have to download any thing else.

As far as Diablo 3 goes, there's virtually no benefit to using a physical installation media. It has the tiny Battle.net launcher on it, and that's it.

However, if your connection sucks as much as mine does, and you don't want to spend a week downloading D3 (or months, if you have a transfer cap!), I recommend finding someone who's already installed the game and copying the game's installation folder to your computer using a USB stick or something similar.

If you point the Battle.net launcher to your friend's installation folder on your computer (using the Locate link next to the big Install button), the launcher will work out what it can use and what it needs to download.

This even works cross-platform: you can copy the installation folder from your friend's Mac and put it on your Windows PC (or vice-versa), and nearly all of the D3 game data will be used as is! The launcher will just download the platform-specific executable. This is, to my mind, the best way to avoid heavy downloads. If you don't know anyone who already has D3 installed, I'd follow @Aequitas's advice and download the game from a PC that has better Internet than you have at home.