I generally only house rules when the RAW are ambiguous or incomplete, or when rules are inconsistent with other rules. The light rules are both ambiguous and inconsistent. What follows are my reasons for invoking the rules listed in the next spoiler down.
1st Ambiguity to be resolved: What happens when light spells overlap with darkness spells? Two cases to consider: 1) when light has a higher level than darkness, 2) when the light and darkness spells are of equal level.
Darkness and Deeper Darkness state: “Magical light sources only increase the light level in an area if they are of a higher spell level than darkness.” I will refer to this as the “darkness rule” in the analysis below.
Daylight spell is higher level than darkness and equal level to deeper darkness. By the darkness rules stated above, it should increase the light level in an area of darkness but not deeper darkness.
Brightest Light spell is higher level than both darkness and deeper darkness, so it should increase the light levels against either of these spells.
Daylight (and by extension, Brightest Light) states: "Daylight brought into an area of magical darkness (or vice versa) is temporarily negated, so that the otherwise prevailing light conditions exist in the overlapping areas of effect." I will refer to this as the “daylight rule” in the analysis below.
Without this daylight rule, the rules for overlap are perfectly consistent: when light and dark overlap, the darkness prevails if it is of the same level as the light spell or higher. The light prevails if it is higher spell level than the darkness.
Note: it is still not clear how to determine the resulting light level. This will be addressed separately below.
With the daylight rule, we not only have it contradicting the rules stated in darkness and deeper darkness, but it introduces a 2nd possibility for the overlap: cancelation of both effects, leaving ambient levels in area of overlap. This adds inconsistency to the ambiguity.
The daylight rule makes no mention of spell levels and so makes daylight both more powerful than other higher level light spells in some cases and less powerful than lower level light spells in other cases. It is more powerful than 3rd level continual flame spells, which don’t cancel or overpower deeper darkness. It is less powerful because a 3rd level continual flame spell will overpower a darkness spell but daylight (same level) only cancels a darkness spell. It is also more powerful because it can cancel a darkness spell of equal or higher level (deeper darkness, heightened deeper darkness spell, or blacklight).
No rational is stated or implied for why daylight works so differently from light and continual flame when they otherwise work in much the same way.
2nd Ambiguity: The rules do not state how to resolve cases where other evocation (light) spells which use light to deliver damage are applied to targets in an area of darkness. Does the darkness block the light and so prevent the damage? Does the relative level of the light spell to darkness spell matter?
Only one spell, sunburst (level 8), addresses this. It specifies that sunburst dispels any darkness spell of 8th level or lower. In this case, a light spell of equal level to the darkness doesn’t just prevail over the darkness but dispels it, even though the darkness rules state that darkness prevails if the light spell is of equal or lower level than the darkness, and sunlight only cancels darkness even if the darkness is higher level than sunlight.
What about spells like flare (0), flare burst (1), glitterdust (2), burst of radiance (2), blinding ray (2), searing light (3), sunbeam (7), prismatic spray (7), and prismatic sphere (9)?
Trying to infer how these interact from the rules on light and darkness, we could use the darkness rule and state that light spells of an equal or lower level than the darkness spell do not affect targets in the area. What about sunbeam, which presumably uses the same power as daylight? Would it be ineffective in any darkness region because daylight only cancels the darkness but doesn’t raise the light level in them?
Another ambiguity is determining what the resulting light level is when a light spell is of a higher level than a darkness spell. The darkness rule states a light spell of a higher level than the darkness “increase the light level” in the area affected by the darkness. It has been clarified in the forums that the increase or decrease of light levels only applies to the ambient light level of the area. Mark Seifter interprets the situation as follows: the higher level “light spell has its normal effect, as per the spell. Do not reduce its light level again for the darkness spell; that already happened.” It is not clear why he assumes that, but it seems reasonable and simple.
This clarifies these sentences in Core Rulebook: You can touch up to 6 willing targets as part of the casting, but all targets of the spell must be touched in the same round that you finish casting the spell. If the spell allows you to touch targets over multiple rounds, touching 6 creatures is a fullround action. When the spell cast requires just a standard action to cast, the caster can take a move action and touch up to six targets, provided the caster can move within reach of each target. This is a special version of a Full Round action, not an additional full round action after the round the spell is cast.