the digital in is specifically useful if you have an A/D...or want to dump data from an external digital recorder without a computer interface. It's probably sufficiently weird most people wouldn't need one tho.
In looking more closely at the new macbook, it appears it's also lost the optical input. There's now 1 port, a combined optical out, headphone out, analog in. It's possible you can also do digital in, but they don't talk about that on the specs page.