Oni said:
However some of the metamagic feats become drastically better when they can be used over and over again. (for instance an extended melf's acid arrow.)
To double extend melfs i need to be able to cast 4th level spells. That means the melfs lasts for at a minimum 3 rounds base.
The first extend takes that to 6 rounds.
So the fourth level spell, double extended melfs, will get any gain at all from the second boosting IF and ONLY IF the target survives for more than 6 rounds after first being hit with it. So, assuming i hit it on round 1, it must still be around into round seven in order to start feeling the AWESOME POWER of the second extended and then at 2d4 per round thru 9.
I have to ask... you have seen this prove to be drastically better in actual play?
Or was it the third extend (which would kick in 13 round after being first hit) that came up impressive enough to be considered "drastic"?
heck, a frequent comment made by a sorcerer player in my game was "melf's acid should be considered a death spell. Why? because everyone i have hit with it died in the next round."
Once an enemy starts getting targetted it tends to not survive for that many rounds so as to make even a singly extended melfs acid arrow worthwhile.
Sure, on paper the "full increase of damage for each extend" seems great, but in practice the slow pace of this extra damage makes this a wasted gain.