Seal of Insight has a proc rate of 15 per minute, not every hit. As such, haste and Reckoning doesn't change the amount of procs.
It does with Mending, though. Reckoning does work with Mending (so does Hammer of the Righteous, or even Consecrate. From reading up on WoWhead, it sounds like _anything_, literally ANYTHING will proc Mending).
Seal of Insight and Mending together are pretty powerful.
Sure, a DK could grab Mending, but 1). He's using a 2H Weapon, so less procs, and 2). He's only got what, one DoT (unless he throws down DnD) and one AoE (well, 2 if you count Heart Strike, but that only hits 2 mobs)?
Paladins are going to see a lot more Mending procs than DKs would, I'd think. And, to top it all off, they Also have Seal of Insight to buff the amount of passive self-heals received.
Edit: I see you looked it up as well.
Yeah, after I posted that, I was like "wait a minute... I should check that..."
Sorry about the ninja edit.
Oh, and BTW, while fighting low-level trash (such as my Heroic Halls of Reflection example), casting Holy Light with the Crusade buff up is also very powerful, especially if it crits. I don't think H-HoR would have been possible without me and mom using up our Crusade charges even while fighting trash. There's also a few times when it is possible to cast Holy Light (with Crusade up) on bosses that have adds AND spells they cast. You're not taking any incoming melee hits while the boss is casting and you can sometimes get out a 25k+ Holy Light out while the boss is casting. This only works on Level 80 content, though, as you don't get Crusade from grey mobs.
You CAN, however, get out the occasional Divine Light. Seal of Insight makes sure you're not going to OOM. I've done that on a few spellcasting-heavy mobs/bosses as well.
Yet another edit: Flash of Light can work too. Edited, Nov 1st 2011 8:16pm by Lyrailis