The way how magical damage works, pretty much every spell currently do close to same base DPS outside burst. Instead of going for a long blahblah magical damage formula this, blahblah magical attack that... I'm going to leave this part about mdmg very short and just say that pretty much any spell that doesn't have a chance to activate eruption/outburst (with M/P reason is even bigger) or some random class specific elites or extra effects here or there are not really that much worthy to cast at all over long time.
If monsters die fast with your party and you like to shine in scrutinizer, you can use some instant even if it doesn't change anything or just ignore monster and be... really useful mage cheering your mates and giving them mental support from the back row in which I have quite a lot of experience.
Elemental Weakness can be very nice boost and those MDEF reducing skills seem to be somewhat underrated opposing to matk increasing skills. Plasma arrow with yay 1% extra critical chance here or there doesn't really make any difference in the end with crits varying a lot and being randomish withn range...
There are some spells like Electric Bolt which will overdamage flame (but only without outburst) with base casting time when all dots are included and used casting time is compared to overall damage output, but whetever it's worth to cast over outburst is another thing specially for M/P... M/wl(?) skill which hits twice can be nice...
You could compare 2,5x instants vs 1x Flame to see if their dmg is more because that is closer to reality. I'm not sure how much exactly GCD is at the moment, but I live in belief that it's around 1 second. This would mean that casting 3x FB would take 3s and compare to one flame which is not entirely true, because pets, runes and some other buffs even outside burst phase will and can lower Flame casting time to under 3 seconds. This leads to more realistic 2,5-2,8s flames even outside burst to compare and match to 2x instant and 0,5 instant dmg of same spell. Instant DPS of course goes higher than flame if you have casting speed slowing debuffs on.
Sadly for class with biggest skill mana pool to use for skills and big arsenal of skills, way too many skills are useless most of the time... Instants hits low and have high CD. Even if using one another skill for minimal DPS bonus might actually still lower DPS because it takes typically longer for human to click multiple buttons rather than just concentrating on spaming one button flame. (Phoenix skill must max, imba, not...)