There is no right or wrong here. It really does come down to personal preference. If you feel you can do better with a straight holder, then that is completely acceptable.
Ellen is correct about the edges. But what it really comes down to is the angle at which you hold your pen and the direction you are/will move to achieve thicks and thins. I think, in my personal opinion, the oblique assists with the angle of the nib to the page which helps me apply even pressure. And slant of the letters doesn't really have anything to do with it (in my case) for modern calligraphy and calligraphy straight up and down without a slant. But yes, the oblique is much better at enabling the 55 degree slant for copperplate.
For me, it is the curves and fine hairlines that benefit the most from the oblique. Which I suppose is counter to what many might experience with flourishing since straight holders are often used for off-hand flourishing. However, I see much more shaky lines from other artists who use a straight holder (v. an oblique) for lettering. But that is just perception, not a hard and fast fact.
I think it really comes down to what you learned with. I find people struggle more initially when learning with a straight holder v. an oblique but once you learn to finesse the pen, there isn't much difference.