Where do you get that 50% number? And what is "high level?"
CR 8 and higher, and I went to an app that had the monsters, typed in the restrictions and compared the numbers. All monsters CR 8 to 30 compared to all monsters CR 8 to 30 with Magic Resistance.
You don't see a point to banishing the strongest monster at 50 hit points and doing clean-up before it returns?
As
@billd91 points out, once you have the strongest monster on the field down to 50 or less, it is a hard sell to say you need to banish it, especially since you can do nearly 40 damage in a single strike.
And again, you don't WANT to use Banishing Smite when the monster is 50 or less hp, because then the damage from the smite nearly kills it, or potentially kills it, anyways. What you WANT is to hit a monster with more than 50 hp, and drop it to 50 to 40 hp (any lower and really, you might be better off finishing it off instead of smiting). So, without knowing the monsters precise hp, and without knowing your precise damage, you need to take a gamble.
Hit a monster with Banishing Smite and drop it to 51 hp? No banishment. And that can be as simple as just rolling below average on the d10's. So, AGAIN, Banishing Smite has more hoops to get through than just casting banishment. Saying "it is a banishment" ignores these extra hoops. Because then, even if you drop them into the range, you STILL need them to fail the save. This is not an easy ability to proc.