But then again, maybe casual players should stick to playing Angry Birds on their cell phones at Starbucks.
You classify everyone as either a hardcore rogueliker or extreme casual gamer? What an odd way to view the world...
Keyboard can indeed be faster much of the time, but certainly not all of the time, and is also not necessarily the most comfortable way to play (a mouse has the joy of being one-handed, so you can drink tea and kill monsters at the same time). More importantly though learning a huge list of keyboard commands all at once is simply too demanding of new players, no matter how hardcore or casual they are. Plenty of Crawl and ADOM players won't touch Nethack because the commands are all different and it's a pain to get used to them. There's nothing "casual" about just wanting to be able to start
enjoying a game quickly. Learning to play a game should be fun, not a chore. Otherwise... well, there's other games to play.
An analogy I like to use is that of Excel. Many that have used Excel for years know the fastest keyboard shortcuts to move contents, insert formulas, etc. Those that don't know it so well get by on context-sensitive menus and clicking on the appropriate icons. The experts probably started the same, and built up their knowledge over time as they got used to the best way to do things. Both groups can still get plenty out of Excel though, and it's not intimidating for new users to get real use from. A good roguelike can similarly support both user types, and with the right tutorials and labels on menus can push the newer players towards the keyboard shortcuts over time, without demanding too much devotion to the UI at once.