Try not to be snarky; just because I believe a character is only ready to be hung up when either a) they die or b) their story is done doesn't mean your way is the One True.
In any case, the answer would be still no, not worth it. Whether the total karma you've acquire is 5, 50, 500, or 5000, a chunk that big for a result that small - and a result which can be so easily destroyed in pursuit of your aims - is seriously not worth it. When you're saving up for major expenses, you begrudge every last point of karma, even more than when you need to get a skill from 3 to 4.
What gets me is the problem that if you disrupt (not banish) a spirit via combat, your Astral Index goes up by one. Which means that if you're a particularly good mage-hunter and better with Blades than you are at Banishing, spirits are going to not like you and thus not come when you call. Which, to me, is bogus. That kind of onus should go to the mage who summoned the spirit and sent them into a fight they didn't win. But ... *shrugs*