As levelling a paladin by the Aoe Grinding Method a lot, i'm very interested in all the items with "...When hit, hit for 15 dmg" or "3% chances on hit to be ...", etc.
Specialy interested in shield spike as i bought all epix shields at the AH, i'm trying to equip some shields Spikes on it.

The thing is on this thread, http://www.wowhead.com/?spell=29657#z0z the guy said:

If you are spaming shield block alot to help your damage mitigation, this can help maintain aggro at the same time.
Especially if you have imp shield block. Although there is a 25 second cool down timer on it, so it might only affect the first attack blocked.

Is that true about the 25s CD on shield spikes? or this is totally wrong?

The spike damage will be applied every time you block, which should be quite often as an aoe pally.