I always take a laptop on a cruise for the following reasons:
1) To bring in and edit/organize my digital photos, also those from my wife and son's cameras.
2) To bring in and edit any movies I've recorded.
3) To use to write my journal, which later turns into my web pages for that cruise.
4) To watch DVDs or other movies in those occasional boring moments.
5) Finally, for email and other internet stuff...
However, don't buy internet minutes from the ship - it's too expensive for the speed you don't get. Instead, take the laptop ashore in any port...you can find an internet cafe within a 5 minute walk and pay less than $5 for an hour or so. And many internet cafes also serve cold cerveza!