Sleep doesn't require concentration because it can only affect creatures below a certain HP threshold that scales unfavorably compared to enemy HP (it's good lvls 1-4, then falls off rapidly) and can be broken both by receiving damage and by allies of the sleeping creatures waking them. Sleep is used to take enemies out of the fight to deal with *other* enemies first.
Hold Person does require concentration because it's a more powerful debuff. Individual hits do not break the effect on the target creature, so consecutive critical hits (which the paralysis condition guarantees on melee hits) are likely and increase in number as the party advances in levels. Because of this, even a single round of this effect on the right creature can be encounter-deciding. Hold Person/Monster are used to set up enemies for focus fire by the party *immediately*.
Shield of Faith and Spiritual Weapon are *very* different spells. Apples and oranges.
But to elaborate, duration-based defensive spells that decrease the enemy's chance to hit you are generally not meant to be stacked. Spiritual Weapon is a compromise to allow the Cleric's "at-will" damage to scale without giving them the Extra Attack feature. To be fair, though, it *is* an actual outlier, flawed comparison or not.
For Blur, see what I said about Shield of Faith. Mirror Image does not require concentration because its effect is diminished by incoming attacks and is increasingly less valuable the more attacks are aimed at you, whereas other defensive buffs provide consistent value.
5e is not perfect and there *are* spells that require concentration when they shouldn't or that do not when they perhaps should. But we are talking about a very small fraction of the full spellbook here, not a general issue.
As for concentration spells being disincentivized in BG3, we are in agreement. But that is purely an issue of Larian's own making. The easiest fix would be for them to dial back design decisions that led to the problem (enemy AI, prone condition not working correctly, too many sources of unavoidable damage), rather than slapping on other solutions that will, in turn, create problems of their own.
Thanks for that explanation. Gives an insight about how it was decided which spells require concentration and which do not. Helped since i do not have much XP with 5e. I still do not like some aspects of it. To eliminate stack buffing there would have been better ways. Having my cleric cast bless and after that not beeing able to use SoF on himself or to protect an ally still feels wrong to me. But i guess that comes from the older games i am used to.
Why i said "maintain" instead of concentration is simply a better wording for it. Same as having hits that do no dmg not be called miss. Gives (or takes) a certain feel. When i try to concentrate and fall prone its not easy to keep concentrating. "maintaining" some spell energy sounds more logical but that is only wording.
