I am having trouble finding this rule in the D20 SRD. Can you point it out for me?
This would be the fact that a person could willingly "drop" any Spell Resistance they had for a spell and could choose to forgo a Saving Throw if they wanted to. This was to stop PCs with SR being prevented from getting buffs and the like, and was very important with regards to Healing Spells which were "Will Save for Half". Undead would try to save against them but living creatures would just let them hit. There was also a Pestige Class in 3.5e that hated magic, and had the downside it HAD to try to Save/Resist all spells - even those most players would "allow to hit".
If we assume NADs are the 4e equivalent of a Saves in 3.5e then we see where this idea of "i want to be affected by this spell" comes from without it having to be "I want all spells to hit me easier".
Not sure I agree this is a valid arguement considering the structural changes in powers from 3.5e-4e, but there you go.
Basically we have a million question on how "hittable" you can make yourself. Can players impose conditions on themselves temporarily and what action would it be to do so, and to undo it? If you allow blinking as a free action then a person can blink as a spell is incoming to grant CA from being blind for that spell. If you allow 'surrendering/rebeling' as a free action you may be able to grant helpless to an attack and then resume fighting straight afterwards etc. RAW never assumed you would want to be hit by your own side, so didn't write specific rules for it, but it may happen.
Consider the following:
If your players wanted to stage a fight to trick an NPC they may want to let themselves be "beaten up" by some people - are they allowed to pretend to be weaker than they are, or do they have to use all their skills even in detriment to their intent?