In a recent review, which was single-blind, I had a paper from a group I know well and the principle author was a heavy-weight in my field, and certainly will review proposals I write in the future. The paper had good content but it was clearly written by a graduate student of his and it really needed work. I made numerous suggestions for revision that I would have felt very hesitant to do if the reviews were fully open (e.g. if my name had been attached). I would have been inclined to simply accept this paper with minor revisions so that the PI did not have any impression I was being unreasonable.
As another example, I had a recent double-open review for a conference where I had to attach my name to a review that was not able to accept the paper. I felt (and still do) that this submission was absolutely not acceptable given the standards of this proceedings. Now, this group has seen my name and associated it with a 'reject', and I am worried that should we meet in the future, this could cloud our interactions.
For both, I've thought about this a great deal and I stand completely by my reviews, my criticisms, and I did try to come across as professional as possible. But I would have preferred to be anonymous, and I see no reason my name being attached would have helped the review process.