Why is LRD 5 level only?
+ (3 to 6)d45 max damage vs. 3d37 at max power
+ smite targeting vs. beam targeting
+ trivializes branches: kills skulls & bone dragons in crypt, kills almost everything in Vaults when metal walls are present, makes Elf (especially the end vault) a cakewalk
+ instakills many 'tricky' enemies like statues, golems which have no other weak points
+ doubles as wand of disintegration in a pinch, makes shortcuts in the Labirynth
+ 1 school vs. 2 schools, 2x less experience invested for the same spell power
+ kills things in your LOS and even further, but fireball does it only with Vehumet
= both are 5lv spells
= both ignore EV
- AC is applied thrice vs. AC is applied once
- useless in open areas if you don't have petrify or there are no skeletons around
Lee's rapid deconstruction should be level 6, or 5lvl Earth/Transmutations spell.
To comfortably cast level 6 spells, you need to raise your schools to ~15 level. To cast 5 level spells, ~10 level is needed. Instead of raising a skill from 10 level to 15, you could train another skill from 0 to 10.
The amount of utility you get from LRD is incomparable to other spells of the same level. You can even cast it in plate mail if you have decent str and armor skill. Doing the same with 6 level ones is much harder.
BTW. is there a noticeable difficulty spike, in terms of invested experience and spell success and spell hunger, between 5 and 6 level spells or is it just me?