Vanguards (DPS and tank) get some very minor healing when they take AOE damage - 2.5% of total health, 3s rate limit. That's an average of 6s-ish to proc the healing relic. I haven't played a shadow and don't know how frequently their self-heals actually happen, but that puts an upper bound of it being 30% more valuable to Shadows than Vanguards *while taking regular AOE damage*.
It's not bad idea per say, but you would really need to examine the actual frequency of area attacks as well as what kinds of attacks are defined to fit the criteria for the heal proc. For example, in a brief look at some logs from last week, the heal activated only 13 times on Dash'roode, 12 times on Thrasher, but a whopping 35 times on Titan 6. The first two cases don't give me reason to believe that the relic would be proccing frequently enough, but there may be fights like Titan 6 where that frequency would be significantly higher.