wowaI remember the times, when the 4 casts of Dimension Door spell by a single Hero were allowed per turn. And those rules were used in considerable club champs for a long period of time.
Only Heroes playing via Internet have led to the restriction of DD spell, and finally the conclusion, that the 2 DD's per turn with expert Air magic secondary skill is optimum case on eXtraLarge maps, was made. Otherwise the DD would have the greatest power, of cause, but on the other hand, with a single jump per turn, the Air magic wouldn't be populal as the skill for the Main -- the only profit from expert one would be the contra for mass Slow in the Final Fight, but expert View Air can be easily done by the Scout. So the 2 DD's are also appropriated to reinforce the Air magic school, which have become less pupular after the previous steps against DD 
Maybe people played with 4 DDs allowed once, but on other forums like Toh, Loh, wcl, etc, etc. DD was not allowed at all.
I would still take air with only 1 DD allowed, as I do on L maps. No fly penalty, haste, view air, and in some cases lightning and chain are all usefull. Its still better than fire and Water.