I agree the the level is Loremaster is pretty power-gamey! But I like it!
Lets take a look at how I would do an Arcane Trickster vs a Rog/Wiz. Both of these are on a 30 point buy with the following starting stats: Str 10, Dex 16, Con 12, Int 16, Wis 10, Chr 10.
Rogue 10/Wizard 10
BAB: 12
Hit Points: 73 (assuming average hit points)
Skill points 152 (not counting Int Bonus)
Saves: For 6, Ref 10, Wil 10
Highest Spell lvl: 5th
Sneak Attack: +5d6
Other gains: Rogue Special Ability; better familiar; extra wizard feat; can't be flanked.
Rogue 5/Wizard 5/Arcane Trickster 10
BAB: 10
Hit Points: 68 (assuming average hit points)
Skill points 132 (not counting Int Bonus)
Saves: For 5, Ref 12, Wil 12
Highest Spell lvl: 8th
Sneak Attack: +8d6
Other gains: Ranged Legerdemain 3/day; Impomptu Sneak Attack 2/day
So by taking the Arcane Trickster you are a slightly worse Rogue and a
much better Wizard, plus your sneak attack is greatly improved. What you give up is a few hit points, some skill points, a few points of BAB, a wizard feat, some familiar enhancements and your 10th level Rogue special ability.
As far as I am concerned, the improved Sneak Attack and the ability to cast 6th, 7th and 8th level spells
more than makes up for any loses.
(edited for formatting)