Ultimately, data integrity is the same thing as data security. If you cannot trust your data not be detectably corrupt in the face of a malicious collision attack, you do not have data integrity. Collided data can be used to cause a DoS, to overrun buffers, any number of nasty things that arbitrary user data can cause when trusted implicitly.

You are making assumptions that are unwarranted. There are other uses for MD5 besides data integrity and data security. For instance, I generate an MD5 hash of a user's email address and use that hash for Gravatar. While someone who knows quite a bit about MD5 and the other person's email address, can force a collision ... all that would get him would be the other person's avatar image ... which is public anyway. In other words, using MD5 is perfectly fine in situations where collisions don't pose a serious problem.