DPS does have diminishing returns in a lot of scenarios. Let me give you an example I posted earlier:
People enjoy the semi-AFK killing of single static respawn mobs.
https://wiki.project1999.com/Travis_Two_Tone according to the wiki he has 875 HP, and respawns every 6 minutes, 40 seconds.
If you are doing 30 DP, you will kill him in 29.1 seconds. This means it takes 429.1 seconds per respawn and kill. 3600 / 429.1 = ~8.4 kills per hour.
If you are doing 31 DP, you will kill him in 28.2 seconds. This means it takes 428.2 seconds per respawn and kill. 3600 / 428.2 = ~8.4 kills per hour.
You would need to kill him for 53 hours straight to get an extra kill lol. This means no breaks in between. You lose your progress if you stop. Extra DPS is not always increasing kills per hour. The 1 kill per hour estimate I gave earlier is assuming you are going balls to the wall in terms of your killing process.
You are assuming every player is going to be XPing in the most efficient way possible, and thus maximizing their DPS at all times. It is a bit silly to make the claim that this DPS difference is going to be noticeable for most players, who are probably not XPing at maximum efficiency all the time.
Max mana can save your life. I have had situations where I ran out of mana, and needed to use my Blood Ember Greaves as manaless FD. Since Iksars cannot do that, they will find extra mana useful.