Extension was included so that a priest could cast a spell on a member of his cult and send him off on a very dangerous mission or heroquest - I.e. give lower skilled PCs the chance to do something heroic above their normal level of capability. It is supposed to encourage and emphasise the mutual reliance a character has with their cult.
If a divine magic user wishes to permanently extend a personal spell, then great for him, but he's ultimately short-changing his own magical capability by doing so. Want a permanent Truesword or Sureshot? Go for it, but don't then complain about getting buggered if you didn't have enough Dedicated POW left for Spirit Block, Heal Wound, Dismiss Magic etc.
Its the typical Gloranthan situation as repeatedly shown by Greg in his stories, that you can make yourself awesomely powerful, but the more you boost yourself the less flexible you become. You effectively paint yourself into a corner.
Needless to say, in Glorantha an extended combat spell is probably going to be the first thing taken down in combat.
GMs who are finding the spell unbalancing should reserve it to cults with the Infinity Rune, or someone who has heroquested to be rune-touched by infinity. Or you can take it out completely or make it Priest level - not that its much use personally for Rune Priests.
These are heavy handed approaches of course. You could simply run a series of encounters where there is no combat, and the permanent Sureshot becomes a millstone whereas those other useful spells available from your cult would have been perfect...
YEGMSMV (Your Evil GMing Style May Vary) :wink: