This is connected to the issues of martial/caster inequality. There's been a lot of those discussions, and there is a thread about suggested fixes, but since this is a pretty unique one I haven't seen mentioned I thought starting a new thread would be OK.
The basis of the idea is that maintaining many of the buffs, debuffs, and conjurations would require some basic form of concentration; a free action every turn, and usually nothing more than that.
Casters (especially arcane) mostly seem to be designed with the goal of being artillery; lots of powers, but easy to shut down. Glass cannons is a word for the extreme versions of this. The issue is that due to the plethora of spells that increase the defensive potential, from displacement and protection from arrows to fly and dimension door, that's not really the case, especially at higher levels.
Requiring a free action to maintain spells could go a long way towards this, since even a single turn of being denied actions would shut down most of the casters spells. This would also be a boost to the currently under-performing monk, who seems to have something of an anti-caster design but isn't really that good at it. Its stunning fist would be a lot better with this change.
It would also mean spells with an instant effect, for example blasts and many other now under-performning spells, would get a boost since they don't suffer from the spellcaster being shut down for a round.
This idea could of course be expanded, and here I had two ideas:
1. Some powerful spells, especially debuffs/SoSs, would require a move action to maintain. This would end the standard for casters to both move and cast every turn, without having to remake all spells into full round casts.
2. Taking damage in combat would require a concentration based on the number of spells the caster has to maintain, or concentration is lost. The DC could be pretty low to begin with, something like 2 * number of spells + damage. This would require some additional bookkeeping, because you have to know the number of spells active, but it's usually not a huge number (especially not with this rule) and would mean that the 13th level caster can't effectively have 10 low-level buffs up at the same time, lessening that sort of bookkeeping and making the all-day-15-buffs sorcerer more risky.
Yeah, so that's the idea. Is it great or just idiotic? It would be nice to get some feedback on this, since to me at first glance it seems a great solution; the effect of the house rule increases by level (which is a goal, since the disparity isn't as much an issue at level 3 as at level 17) and it's not really that much rules that has to be changed. The only issue is finding out what spells should require the free action to be maintained (and the move action if that's a desirable idea).
This is obviously going to be quite an unpopular change. It encourages a shift towards what is currently 'inferior' paradigm (blasting), and heavily burdens highly effective tactics. What worries me is that it also penalizes the more cooperative behaviors of casters (boosting the effectiveness of others) in favor of more "selfish" instant/artillery spells.
I can see tacking on the rider "concentration" to what you perceive to be the most offensive of SoS or buffs, but trying to implement a maintenance plan as a blanket and across-the-board solution for all durationed spells is a direction I don't think anyone would want to go. If you really want to try, I recommend offering small class abilities that allow a slow "trailing off," such as a Transmuter's spells lasting an additional 1d4 to 1d8 rounds after losing concentration (unless normal duration expires first), or perhaps an Arcane Bloodline Sorcerer being able to retry a failed concentration check to maintain concentration on one spell of his choice (despite losing all of the others).
This also begs the question of some magic items like potions, scrolls, wands, staves and wondrous items. Would they count as maintaining, or would they get a free pass? (this brings to mind a picture of a spell scroll slowly crumbling to dust as the duration of its spell counts down) This could have a serious impact on the price of spell items.
Careful, my good sir. There are plenty of casters out there focused on utility. Saying that arcane spellcasters are intended to function as artilery is a gross generalization.
Yes, it's a popular archtype, but not every fighter is walking around with a longsword and a shield, and not every cleric is a living case of bandaids. Bards are arcane casters too, and I know the witch I'm playing right now would be enormously affected by such a change.