My thinking was a top flight CB would give us a lot of options.
If Henry gets dinged up again (it's been 2 consecutive years running now), we'd have a replacement. Or, if the rookie can come in and play right away, we could always move Henry to FS, for example.
Also, this is probably Glenn's final year as our nickel CB (I'm guessing) given his age. So, we'd have some options in place for that contingency as well.
I just think that given this is such a deep WR class, a good one could be had (and developed) later on in the Draft.