MarkB said:
Because a swift action is a special form of action that lets you get a large benefit out of a very small investment of time - a benefit like a spell, or the ability to change the way you use a weapon, or any of a variety of things that are valuable in combat.
Being able to do it only once per round is the cost of that benefit. Reducing that cost should not be done lightly. Your solution is to pay that cost by trading something - your standard action - that you weren't going to use anyway. That isn't a fair transaction.
Simply from the balance perspective, the real issue is whether a swift action is
generally more valuable than a standard action. There will, of course, be
specific instances where a particular swift action is more useful than any other standard action that a character could take.
However, there will also be specific instances where a particular move action is more valuable than any other standard action that a character could take - for example, a character on the other side of a door from an large group of orcs may decide to move to the door (a move action) and close it (using his standard action to take a second move action) instead of taking another standard action. Yet, nobody seems to be arguing that trading a standard action for a move action is unbalanced.
Unless it can be shown that a swift action is generally more valuable than a standard action, I am not convinced that trading a standard action for a swift action is inherently unbalanced.
1. Quickened spells already have an inherent cost in terms of the higher spell slot.
2. The channeled spells from PHB2 that could be cast as either a swift, standard or full-round action have a reduced effect if cast as a swift action.
3. Many swift spells and abilities that enhance a spell, attack or some other ability last only 1 round, so casting another enhancing-type swift spell is usually a sub-optimal decision (following up with an offensive swift spell is a possibility, but see points 1 and 2).
4. Most immediate action spells are defensive in nature, and while I can see how being able to roll out defensive abilities twice in one round could be useful, I don't see it as being inherently unbalanced.
If the problem is with a specific swift-action spell or ability, it seems to me to be simpler to house-rule that the specific spell or ability cannot be used more than once per round. Naturally, YMMV.