Dion is a solid defenseman and all this talk about his play dropping off is, for the most part, ********. His play isn't the reason his stats have gone south - it's his usage.
This year with the Leafs he's started 39.30% of his shifts in the offensive zone - that's over 60% in the d-zone. He's basically being Boyd Gordon'd.
2013-14: 34 point pace, 39% o-zone starts, top competition
2012-13: 48 point pace, 41% o-zone starts, top competition (shortened season, high shooting %)
2011-12: 44 point pace, 50% o-zone starts, top comeptition
2010-11: 37 point pace, 49% o-zone starts, above-average competition
2009-10: 32 point pace, 55% o-zone starts, above-average competition
2008-09: 48 point pace, 58% o-zone starts, average competition
2007-08: 60 point pace, 57% o-zone starts, faced average competition
He's dropped from 58% o-zone starts and average competition to <40% o-zone starts and top competition.
So given a reasonable zone start rate (around ~50%), there's a good chance he's a 40ish point defenceman that can play against the top competition.
It all depends on cost but I'd gladly take him.