If you need an in game reasoning I'd say that Mentalism is the (internal) power of the mind, while Essence and Channeling are more external power sources. I guess it depends on your point of view about what Mentalism is, but I look at it like psionics... it's not something you are taught, it's something you figure out yourself (it's, literally, in your head), it's not a source of energy like the other realms.
How could you draw mental energy out of something and imbue an item with it? Of course, if you had an evil mentalist that was taking the mental energy from people (i.e. killing them) you could come up with some interesting items...
You know, that's a pretty darn good explanation. I'm sure we could all come up with a myriad of explanations as to why Mentalism doesn't have or why it could have Lord Research. I like the explanation you came up with, and I'm going to stick with that should the issue ever arise in my campaign. I don't see that ever being an issue though. A level 25+ spell caster in RM? Bahahahahahahahahahahahah.
Although, couldn't a Mentalist research one's own mind and find ways to do more incredible things with it? I would love to see the failure rolls for exploring within your own mind at 25th level, then fumbling a dice roll whilst inside there. Ohhhhhhhh! Mentalism Lord Research Spell Failure Crit table, anyone?
A-Crit: Target is amazed at the complexities and intricacies of his own mind. Gets lost wandering the maze of possibilities. Target is conscious, but only shows facial expressions as he discovers himself from the inside out. "Waking coma" for 1+1D4 weeks.
B-Crit: Target gets lost in own mind. Target believes and interacts with the real world as if it were the imaginary one that exists in his mind. Only a "death" event in his dream world will bring him back to reality. GM should feel free to create adventures within target's mind for him to go through. Time flows normally for the target within his mind. When he comes out, only a few seconds have passed in the real world.
C-Crit: Target pulls his own essence into his brain and is trapped within the infinity of his thought. Appears to have permanent coma to outside observers but living happily within his own mind. If target dies in his imaginary world, he dies in reality too.
D-Crit: Target wanders aimlessly through the infinity that is his own mind. Target is in coma permanently. Only an Evil Mentalist with Lord Research skill will be able to draw the target back to reality. Target's body ages and atrophies as normal while in coma.
E-Crit: Target sneezes while inside his own mind. Target believes the force of sneeze destroys vital areas of brain. Foe convulses violently for 1 round then dies. Get a tissue.
With that being said, the world that I run is strictly an Elementalism world. The only magic system/realm/convention is what is laid out in Elemental Companion. It has enough variation to give the players a good amount of freedom, the spell lists and number of spells are manageable for GM and for the group of newbies whom I am introducing to RM in a couple of weeks, and I absolutely love the approach the writers took with Elementalism. Besides... GM's prerogative as to what is available in the world and what isn't
Cory, your theory is the one I go with.... until you're killed or I find someone better. *
*-Starship Troopers.... Just in case someone gets the wrong idea and thinks I don't like Cory.