OT: D&D and multi-classing.

[Taz-in-Space]

Ouch! Advancement by levels where experiance is the same for all levels. That sure would make multi-class characters weaker to play.
When I played AD&D there was a increasing requirement to advance to next level. That a least allowed multiclass some usefulness.

I remember playing a Fighter/Thief and being useful to the group as a Thief that was also handy in a fight. (I remember I used to take the trailing position in the formation to guard against attacks from the rear.)

With the rules as you stated, I would not have been as effective.