I think most people agree. If I wanted to keep a spell up for a whole year it would probably be something defensive like Protection, or something more generally useful like Bladesharp , Vigor, or even Mobility. Something that will be useful for more than the first round. Stuff that even if someone spends a lot of MPs to blow down, is still worth it because it made them spend those MPs blowing don your spells as opposed to casting powerful offensive magic of their own.
BTW, I take if from your posting then that the Rune Points are locked up as long as the Extension is running, right? So someone can't extend a spell for a year, then pray to regain rune points, and repeat the process. Good. Locking up the 6 RPs in Extension go a long way towards the cost.