DRY makes it harder to actually understand the system as a whole in some sense, since it usually means some indirection has been added to the program. However, it avoids the one thing that actually makes me pull hair out: code that looks the same because it was duplicated but is just different enough to trip you up because each area it was used required minor syntax changes that had major implications for the result.
Repetition also makes it harder to understand a system: not only do you have to read more, you also need to remember and compare repeating fragments that may be identical or just similar.
What makes it easier to understand a system is simplicity. I'd argue that DRY, deployed with a right strategic plan, usually does more to simplify things than does copy-paste.
But, as any tool, DRY is but a tool; to be useful it requires some skill.
