Someone actually took the time to pirate these apps, and then spam the market with links to a site they created to host them. All for free. Just because they want to help you out. Yep, that's right, they did a bunch of illegal activities, for the sole purpose of helping you avoid paying two bucks for an app. Because they like you (but dislike developers). The apps are perfectly fine, because no such person would ever have an ulterior motive.
If you can't spot the sucker in the first half hour at the table, then you ARE the sucker.
There is nothing stopping them from altering the applications to encrypt your files so they can sell you the decryption key...or simply sending their contents over the internet. Or retrieving the contents of your address book so they can pose as you sending messages to your friends to give them proper computer viruses. There are a million and one scams possible with access to someone's personal data on their phone. If you think you're immune because you don't have anything important on your phone, feel free to go find out how you're not.
The apps will be limited to their specified permissions, but many/most apps have the INTERNET, WRITE_EXTERNAL_STORAGE, or READ_CONTACTS permissions.
I don't know how common it is to use pirated apps for nefarious activities at present. But as Android becomes popular and people stay greedy, it'll be a huge market.