I must admit that I tend to agree with SnS: Every time a curry dish is prepared from pre-made base, spices are fried in oil, thus producing freshly flavoured oil within that particualr dish on each occassion. And also why take out oil produced in the base only to add it back again to final dish if it's reused? And as SnS says, if its all lft in the base, provided the base is well stirred (to off-set the effects of any localized separation), there will be equal amounts of the origanal flavoured oil in each portion of base!
It was interesting to note that JerryM felt that a long simmer didn't adversely effect flavour.