Banishing is a lot more effective than people think it is.
Banishing pits your Banishing + Magic vs. the spirit's Force. You only apply the summoner's Magic if the spirit is
bound, which is going to be a rare case indeed. And if it is a bound spirit, it's clear the GM intended it to be a major threat anyway, so no need to cry about that.
Assuming a Magic of 5 and a Banishing skill of 4, that's 9 dice for you. Even against a powerful Force 6 spirit, you have the advantage. On average, the spirit will get a whole 2 hits versus your 3, meaning you reduced its services by one and suffer a devastating Drain Value of 2. If you can't completely negate a Drain Value of 2, well, you probably deserve to die because of it. And this ignores your use of Edge (and while a spirit can do the same, I doubt most summoned spirits will want to stop you from sending it back home and/or releasing it from servitude enough to waste its own Edge), the use of Banishing Foci, and various other considerations. Yes, it's not going to be a one-hit one-kill attack, but if your team is woefully unprepared for dealing with enemies with Immunity to Normal Weapons 6, Banishing is the
exact opposite of useless. And it sure as hell isn't deadly to use.
What's more, unless you're in a powergaming group, you're much more likely to face spirits in the Force 4 range anyway, making it even easier to get banish them. Though in those cases a Ruger Super Warhawk with EX-Explosive ammo will do the job better, so it's a moot point.
To drive the point home, here's five quick contests between a Force 6 spirit and a mage with Banishing 4 and Magic 5.
[ Spoiler ]
Mage: 5,5,1,6,6,2,2,3,1
Spirit: 3,5,1,4,1,4
Result: 3 services removed, 1 DV drain
Mage: 5,5,1,5,5,6,5,6,2
Spirit: 2,1,4,4,2
Result: 7 services removed, NO drain
Mage: 6,2,6,6,1,1,3,5,2
Spirit: 4,2,3,3,3,5
Result: 3 services removed, 1 DV drain
Mage: 6,1,5,2,1,1,6,1,5
Spirit: 4,2,5,1,1,4
Result: 3 services removed, 1 DV drain
Mage: 5,6,2,1,3,4,6,1,5
Spirit: 5,5,4,4,1,6
Result: 1 service removed, 3 DV drain
Hardly useless or damaging.