[color:"orange"]what about resurrections? Should being resurrected refill their "XP bag"?[/color]
In DD you could resurrect a fallen opponent, and it would fight for you, though would not follow you unless you cast a spell on it. If you resurrected something and walked far enough (or teleported) away, it would turn hostile and you could return and re-kill it, for experience points and possible loot.
Opponents in DD do not respawn, so this resurrection trick can come in handy if you need to level up, but there aren't any handy monster filled areas around. For example, there are several level bonuses in the game, which will give you enough experience points to reach the next level. If you wait until you just level up before getting the bonus, you will maximize the effect. One of the level bonuses is in a small area which you can only visit once. When I discovered the level bonus, I reloaded, and put off doing that quest until later. I tried to time it so I was just about to go up a level, and that clearing the area would do so, but I didn't get it quite right, and had to resurrect and re-kill a couple monsters 4 or 5 times to level up. If resurrecting was not possible, I would have had to reload again, or accept a tiny experience bonus rather than a full level's worth.
In DD the experience points required to reach a particular level are proportional to the cube of the level. The experience points gained from defeating opponents is proportional to the level difference between your character and your opponent; if you are at a much higher level than your opponent, you recieve less or no experience points. I never checked this in DD, but BD had the same experience points required per level formula, and there if you were 8 or more levels above an opponent you did not get experience points for it.
Between these two effects, resurrecting and re-killing opponents can help level early in the game and assist in specific circumstances, but in general it is slow and tedious. If done with a strong opponent that always drops something, it can be a slow way to generate loot, but money isn't really a problem as the game progresses, anyway.
BD does not have a resurrect spell, but it does have battlefields. These are areas outside the normal game area with some merchants and randomly generated dungeons. The dungeons are completely optional (I only visited the battlefields for the merchants); the monsters there do not respawn, but there are more than enough opponents there if you need to level, or are in the mood for some hack and slash. The dungeons in the BF become available as you find certain keys in the main game; if you wait too long to clear the lower level dungeons, you might not get much (or any) experience points for them.
In general, I don't like respawning monsters, unless they are confined to an area that I don't have to go through again if I don't want to. Allowing resurrection and re-killing is an acceptable alternative if you just need a few more experience points, or a little extra loot to trade for healing potions, or whatever. Realistically, though, resurrected monsters should not keep dropping loot. If they have a random chance of dropping something, and did not do so the first time they were killed, I can see allowing another chance if they are re-killed. Actually, I'd like to see a more realistic item drop system, but that's another topic.
The experience points should at least degrade each time an opponent is killed; perhaps the first time they are resurrected and re-killed they give half experience points, then a quarter, then none. Alternately (or additionally) maybe there could be a limit on the number of times a creature can be resurrected.
With something like the battlefields in a game, re-killing monsters for experience points or loot would be unnecessary, so any resurrection spells should be designed for characters who wanted to either dabble or specialize in a necromancer style of play.
