Thanee said:
I think this is where I see the main advantage. It's especially good with all those utility spells. You can max out your prepared spells with combat stuff and just swap in the utility spells as needed.
In theory, this is correct. In practice, it may not be. You cannot swap into an empty spell slot (sorry, but I forgot to mention that). What this means in theory for Wizards is that they study most of their spells and have few slots open and swap in utility spells as needed. What this means in practice (at least from what I have seen) is that once a slot has a spell in it, that spell often tends to get cast sooner or later during the day (except at much higher levels where you have so many spells). Hence, the higher level spell slots are sometimes not available to be swapped, especially later in the adventuring day.
If the Wizard keeps slots open, then he is often forced to use other, sometimes less desirable, spells to accomplish his goals. In this case, he ends up having to take out the 15 minutes to fill a given slot anyway.
Thanee said:
As a Wizard, I always assume maximum ranks in Concentration, they have enough skill points usually, thanks to their high Int.
Yes, you could easily do that and probably do.
But, with the revised rule (DC 20 + 2 * spell level), what this means is that at first level, your chance to succeed with your best spell with an 18 CON is 35%. At seventeenth level, your chance to succeed with your best spell with an 18 CON is 35%. Even if you throw in the Skill Focus feat and a Bears Endurance spell, your chance goes up to 60% for your best spell at most levels. This does not take into account (in my game) being injured which can drop this chance by as much as 25% (typically in the worst of conditions). So, you kind of have to take both house rules into account.
With more typical caster situations (16 CON + max Concentration + no Skill Focus + no magic dedicated to this), the chances for your highest level spells (at the level you acquire them) drop to the 5% to 30% range (depending on how injured you are).
Sure, high level Wizards would be very good at swapping in low level spells. But, over the lifetime of the Wizard, even one with a maxxed out Concentration, it will fail often enough that most players will not do it on a regular basis, at least for their higher level spells. Is it really worth it to have a 30% chance to swap out your only 9th level spell at 17th level (and take up a round doing it)? Usually not for most players.
I also view this ability as making a spell caster more of a target since injuring them while they are doing this is a double dipper. You injure them like normal, but you can also prevent them from being successful with it (and they lose the spell). Plus, at higher level where the damage done is greater, the chances of success of doing this in combat drops even more (with my other house rule).
Out of combat, sure the chances are not that bad, especially for a casters lower level spells.