They'd be there for multiple reasons. To show that the world (even the MQ areas) aren't centered to the players proficiencies (and of course, they wouldn't be all around the place --- the better, the rarer and harder to get); to provide a bigger incentive to specialise - I always thought it was odd, that the weapons had those requirements when the game still would give them away only when it thought the player is starting to close in the req's by his level - if you're get something, you can not yet use properly gives a visual incentive to the player, much like those RPGs that have hard skill/stat req's and still give away items the player can't possibly use; to provide risk/reward situations (even if not able to use them, they provide a good amount of cash - or they could be saved for further use with the expense of inventoryspace). So I guess it is about earlier specialization - and the 'spasmodic' leveling - if I understood what you meant with it - is the players choice, if he is able to obtain a greater level item early on in the first place - finding AMR or assault carbine "too" early still leaves melee, unarmed, enegry weapons and explosives and their "big ones" undetected and unexplored (hasty leveling might just lead to an unwanted result - not to mention neglecting other skills, which too would carry more weight).
And I want to press that with the earlier availability of better items I don't mean easy access or all over the place availability to pick the spoils of others' war.
I didn't think you meant easy access, I was just concerned about having a repeat of FO3 starting scenarios where one can rapidly outstrip the 'power curve'.
Many game with 'hard' usage caps are class-based, so you will know immediately if you will ever have a use for a lucky find of a powerful item; however, Fallout games are more free-form in character design, so my concern was that finding multiple classes of good loot early on would lead to a player dramatically boosting first one skill and then another rather than sticking to his/her original concept.
For example: say I decided to make a Gunslinger tribute character (main character from a series by Stephen King) with Fast Shot trait, Quick Draw and Rapid Reload perks, max Guns skill, and so on. Said character opens a lock for which he barely qualifies after magazine and chem use, and finds a Displacer Gauntlet. My immediate reaction as a player would be to want to pump my Unarmed skill, because I know that with this item I can obliterate almost anything short of a Deathclaw or Giant Radscorpion, however that is completely contrary to the character's concept and the skill points would basically be 'wasted' in pursuit of power in a field I was originally intending to ignore completely.
Now, if what you are suggesting is that rare finds should be keyed to a character's primary skill then that's different, since in that case said character might find a Hunting Rifle or Trail Carbine and want to pump Guns more heavily early on than might otherwise be done.
Basically, what I am saying is that if a character manages to find 'rare drops' from multiple combat classes, then you might see a push in Guns of 15-20 points followed by a similar one in Unarmed, and so on, rather than a steady growth in that character's chosen fields.
I do agree that it's somewhat 'anti-immersion' to have the quality of loot improve as the character does, however at the same time that's an ancient RPG staple- you're not going to ever find, say, a +6
Broadsword of Slaying on a level 4 Fighter-type, as it's high-end loot that is only ever found guarded by really nasty creatures which would shred a low-level character in short order. As such, it serves as a 'power limiter' to prevent the very 'overgrowth' you mention that could cause problems later on.
On the other hand, if you know there are better weapons coming at higher levels there is incentive to stick to one or two combat skills, so that when you
do get access to the higher-end stuff you can actually use it properly. Of course, the 'Tag!' perk means you could readily develop three instead at no real cost, and it is also feasible to spend levels 2-5 cramming one skill full of points and hitting 100 far earlier than one 'ought' to, so I suppose there's something to be said for letting a few items be able to be found a bit 'early'...