here is a screenshot of me during a t2a fight casting heal.(proving that what we use is kinda accurate on dmg healed).

Note that this is the ONLY screenshot that i have out of around 2000 where i am casting a heal or being healed by the heal spell.
So why didnt anyone use heal? Not because it didnt heal good dmg but most probably because there was a big recast delay BIGGER than the current 0.75 second we use. most probably around the casting time of an 8th level spell(2.25 sec). That meant you coudnt spam heal or ANY spell for that matter unless you waited that 2.25 sec. Then you could recast instantly a spell after releasing the first one and fastcast it negating the 2.25 wait time you had to hold your spell.
Now the only source of something like that I have is a UOPG article written during uo:r on how to fastcast and negate the recast delay
http://www.uopowergamers.com/e-pvptapion.shtml
note that he says "old fireball days". I presume he means the dread days before t2a. that would also mean that the delay was added for t2a or during t2a....OSI made the limit so you cant cast spells immediately but with double casting the first spell is delayed so the second spell can go fast hense by passing the delay that OSI set to prevent people from casting several spells in a row like the old fireball days).
In order to double cast you need to cast the first spell and wait about 2.2 seconds after the target cursor comes up then target cast the next spell immediately then target again. Its a good idea to hold down the 2nd spell after you push the macro for the first spell (thats why I stressed earlier that its important to have offensive spells on the F keys so you will be able to push the target for the first spell after waiting 2.2 seconds while holding down the second spell). Lets experiment now to see if you have it right...find a dog cat or anything that will survive 2 mindblasts and set him on last target....
now its an assumption but at least its something because heal spam did not exist during the t2a era.