Yeah, its pretty good when you absolutely must land a spell. Heighten + bend luck makes it almost certain to happen. Plus, there's a certain amount of satisfaction when the monster makes a save by one or two only to fail when it comes into play.
No, nobody is arguing that plus points aren't good.
The issue here is the cost. Bend Luck is horribly expensive and stingy in comparison. A single d4 simply can't change much of anything, especially since you don't get to know by how much the enemy made its roll.
Look at Bards or Battlemasters or Warlocks to see how its done right. In fact, let me illustrate:
Bend Luck (I): Starting at 6th level, you have the ability to twist fate
using your wild magic. When another creature you can
see makes an attack roll, an ability check, or a saving
throw, you can use your reaction to roll 1d8 and apply the number rolled as a
bonus or penalty (your choice) to the creature’s roll. You
can do so after the creature rolls but before any effects
of the roll occur. Once you use this feature, you can’t use it again until
you finish a short or long rest.
Bend Luck (II): Starting at 6th level, you have the ability to twist fate
using your wild magic. When another creature you can
see makes an attack roll, an ability check, or a saving
throw, you can spend one or more sorcery points to roll that many d6s and apply the total rolled as a bonus or penalty (your choice) to the creature’s roll. You
can do so after the creature rolls but before any effects
of the roll occur.
Bend Luck (III): Starting at 6th level, you have the ability to twist fate
using your wild magic. When another creature you can
see makes an attack roll, an ability check, or a saving
throw, and makes or fails its roll by less than 5 points, you sense this, and can spend your reaction and 2 sorcery points to change the result from failure to success or vice versa. You
can do so after the creature rolls but before any effects
of the roll occur.
I: This version has a lower cost (your reaction only), but can't be used more than once per rest. Do note the bonus is still twice as high as in the original; much more in line with Dark One's Own Luck or Bardic Inspiration.
II: This version costs Sorcery Points, but allows you to potentially change a much larger range of results. It also does not cost you your reaction.
III: Here the cost remains the same, but the
uncertainty has been removed: just the fact that you
know your investment will have effect, that you won't roll badly and just waste your effory, is a huge bonus.
I stand by all these three variants as reasonable. You should easily see that the book version is much much too expensive for what you get.