From a narrative standpoint, what is being gained or lost by allowing a generic prolonged spell?
Say you put a rote block on someone in exchange one, and now extend it in exchange 2.
You lose the chance for failure, which is possibly dramatically important.
In my experience, if that roll did fail, the player will then spend a fate point to keep that failure from happening: this is the type of roll that if you miss it, you tend to miss it by only one or two (being you rolling unopposed against a difficulty that you're setting, you basically only fail by more than that on a really suck roll or by setting the difficulty too high). Thus, it's a perfect fate point expenditure, and appropriate for a dramatic moment.
But extending a spell itself isn't as narratively cool as casting a new spell; it's really just locking it down a little better. And if someone is devoting a rote to it, they are saying "I've got this down pat, i plan on using this spell multiple times in a session, and this is an important part of who my character is and how s/he relates to magic."
So, by allowing a generic prolong rote, you're letting a player say "my character is the kind of guy who has NO trouble extending his magic for a reliable duration, once he gets the spell up." Which is a cool, useful thing, on par with Molly's perfect wards or harry's extreme fire magic.
However, if it's so good that absolutely every player would want an extend rote (and they might), then it becomes uncool for lacking uniqueness.
So, I'd probably okay it if there was one wizard in the group, or only one person using spells that benefit from extension, because it's a more powerful statement about the character than the drama I would get out of making the roll happen.
If there were multiple wizards in a group, I'd probably lock it down to the same type of magic, as the first example showed (an earth magic extension, ect), to keep some uniqueness.
