Griffey has lost a step in the field, and Reed is a much better defensive option. Rate has Reed at right about league average (2005 Rate=99, 100 is average), while it has Griffey as a liability (2005 Rate=87, 12 runs per 100 games worse). Prorated over 130 games, Griffey is about 16 runs worse than Reed. However, Griffey makes up for that with his bat, and then some. He beat Reed in RARP by 40 runs (48.6 to 8.5).
I don't think a Bernie comparison is warranted because Williams has never been the caliber hitter that Griffey is. When some of Bernie's talent declines in the field, then he truly becomes a liability because his offense isn't enough to carry his defense. That is obviously not so with Griffey.
Regarding the future, Griffey actually does make more sense as long as he doesn't cost any of the top prospects. Reed is someone on the way up, so Seattle is justified in asking for premiere talent in return. Griffey is on his way down, and Cincy is looking to get out from under his contract. Clement + $$ is fair. I don't know if that gets the deal done, but I'd think Cincy would have to consider it long and hard. A two year rental of a no-glove, all-stick CF would give the farm hands enough time to finish their development.
Is it a risk? Absolutely. But, as long as it only costs money and not some of the kids, then the reward end of the deal could be huge. It will give the Sox the worse defensive OF in the bigs, which isn't good with a staff of flyball pitchers, but they won't struggle scoring runs. And, that has worked in the past.