I'm not 100% sure where rationale's collected, though we've had a few trap discussions over the last year, sometimes in connection with NWN1.
Some of the points / factors can include:
- D&D has no portable trap kits. There are huge long lists of the kinds of traps one can craft, found in the SRD, Song & Silence, and other sources, but they are (1) all stationary, (2) take weeks to craft/construct, (3) cost usually in the thousands of gold pieces. There's nothing like the "bomb in a suitcase" concept which the NWN1 / NWN2 implementation seems to focus on.
- AI melee critters in NWN1 & NWN2 are good at one thing: taking the absolute, most direct route possible to their target. Doesn't matter if the creatures have godlike intelligence, 20 ranks of search, or even if they saw you deploying a trap during their charge, they do not alter course if they "see a minefield" right in front of them, they will be charging through it. I'm much less concerned about this with PCs, as PCs can either sink ranks into Search or not, and use their heads about tactics and bottlenecks, wondering if they ought to just charge a sneaky opponent or not.
- D&D seems to draw lines between normal, masterwork, alchemical, and magical items. "Bomb in a suitcase" items we're pretty sure aren't meant to be magical, as non-spellcasters seem to cobble them together and be their most frequent users. Pretty sure they're not normal or masterwork items either, because normal/MW things don't... well... explode and kill a dozen people. That leaves alchemical items. There's a pretty good, tight list of things to consider when asking "so how good is an alchemical item compared to a magical one?", on page 35 of the Arms & Equipment guide. Alchemical items can be interesting, but they're really boiled down to "poor man's magic." If you want a "Delayed Blast Fireball" for instance, get a mage, not a 2000gp bomb. Alternately, get a scroll or other device and invest in UMD ranks.
...
It's on this last point of alchemy vs. magic that I
personally think a couple of the "minor" traps, possibly with altered effects, probably could have a role in the game. That page 35 A&E list mentions things including "temporary shaken or dazzled" may be possible, "small amount of damage to a single creature, or modestly more to a small subset of critters (like a racial type)", "shouldn't duplicate a spell effect / affect a large area."
NWN2 traps listed here:
http://nwn2.wikia.com/wiki/Trap
Again, you'll find that the damage effects even for minor spike are higher than found with any published alchemical items out there, and the "slow" effect of a minor tangle trap is something you'd normally need a 3rd level arcane spell to achieve. Other end of the spectrum, even a minor electrical scripted to effect 4 creatures, 8d6 apiece, again, something normally a Lightning Bolt by an 8th level caster who got lucky with AoE grouping would be needed for.
Note again, my view is a personal one, not speaking on behalf of DMA, the whole standards team, or anyone else. But if the scripts for NWN2 traps are anything like the scripts for NWN1 traps (example: take the short, simple nw_t1_spikeminoc.nss script as how Minor Spikes work in NWN1, maybe the same used in NWN2), they're definitely changeable as custom content, i.e., alternate damage dice, altered "extra" effects one might inflict on failed save, etc. Again, if you want to BLAST the pants off opponents, buy appropriate items, like wands (high cost). If you want to INCONVENIENCE opponents, existing alchemical items like tanglefoot bags, alchemist's fire, or maybe some facelifted minor traps would be the route to go (low to moderate cost).
PS / Self Disclosure: playing a successful rogue in NWN1, and trap stockpiling gives a very, very dangerous edge that I'd daresay a similarly leveled pure-caster (even an evoker) would have a tough time equalling, if we both had time to prepare for the same encounter, or an encounter with each other (ie, talking about human controlled, not AI). Something not right about that, but the horse is well out of the barn in NWN1.