Chad Callihan explains what a dirty read is and does a cost-benefit analysis on it:
When you are not careful with your transaction isolation levels or you get sneaky with the NOLOCK hint, one problem you can encounter is a dirty read. Let’s look at a short example to demonstrate a dirty read.
In a vacuum, I’m not necessarily opposed to the idea of dirty reads because you can find legitimate cases in which they can be useful. In practice, I’m generally very much in opposition because of two reasons: first, Read Committed Snapshot Isolation eliminates the majority of those reasons; and second, because the misuse is almost always in the direction of over-use of NOLOCK hints.