My standard asset is
Death aura
Negative energy burst
Enervation
Ice storm
Dimensional Door
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It would be nice to have pk / acid rain too (there are several higher lvl slots wasted)
I do not see what the argument is, I do not think anyone would argue it would be bad to let spellcasters prepare lower level spells in higher slots. Personally I think its a great idea since it follows PnP and helps players (would even help my ranger), but it really comes down to how hard it is to implement since if its rather easy then devs can be convinced to do it, and if its hard it will never happen because they will never consider it a priority.
Yes, it would be bad. If you are a wiz, you'd put basically all those spells lv4(PK, Enervate, SS, DDoor, Fireshield, FW, Icestorm in the lv5 (because you just need DOT's here as a wiz and maybe CK) and lv8 slot.
We don't need lower level spells in higher slots, we need more and new spells in the higher slots to make even more choices. That's what casters are, that's what I like about casters.
Casters = choices. If you don't want to make a choice, don't play a caster. This is less true for a wizard. You can swap spells at will, make use of that wiz's perk like Impaqt said.
Level 4 is VERY crowded with spells, more so as already pointed out by a pale master.
Moving death aura to level 5 and stepping up lesser death aura by one would help a lot.