I had found out that a lot of people dont know about this so:
At first Leoric signet can look realy awsome because 20-30 bonus EXP looks realy cool BUT if you do the math with other exp bonusis (70% NV bonus and up to 31% radiant star ruby in helm) the actual exp amount increases only from 201% (100% as base + 70% 5NV + 31% Gem) to 221-231% exp so the actual OVERALL EXP bonus reward isnt 20-30 as it may look But only from 9,95% (20% leoric) up to 14.92% (meaning difference between 29% one and perf 30% one is 0.49% overall bonus exp at cost of around 50m gg increase.
Now the tip:
You can also give the leoric signet to your mercenery and it will work same as MF so 20% of bonus EXP will be added to you so from 20% one will become 4% one and from 30% one will become 6% one. And overall increase will be by 1.99% up to 2.98% Overall bonus EXP.
PS: This isnt just theory it was tested with 29% ring on merc and it gave around 3% increase which is correct. and same was tested when wearing on main char that it isnt overall bonus 29% but only around 14%
The only thing which wasnt tested was getting ring both on merc and character at same time but i belive they would simply stack as before thus increasing up to 17.91%.
Now from what i had seen and talked with high paragon level players wizards can afford to drop one ring easily as in high end equip their archon form does enaugh damage and the only thing they lack is speed and so they appriciate the bonus exp most.
But many barbarians unless in compleatly overgeared rather take SoJ to beat mobs faster which gives overall more XP for them than just the bonus.
And second thing the compleate TOP paragon players use or used Leoric's signet on mercenery for the small bonus exp at no damage output cost.