My lowly take on it:
First, 10th doesn't exactly say that crossing is the 100% right way. We cross in spiderweb cuz it offers better control. Or for the x break cuz it just falls into place.
I like crossing my ankles for control. There are a lot of armbar escapes that are nullified by crossing your ankles under the far shoulder. Also, Ronda, the friggin Jesus of armbars, crosses her ankles. She used it to tap Miesha. So, if it's good enough for Ronda, it's good enough for me.
That said, it's more situational than a 100% must every time. But it's definitely not a bad thing to do when it's appropriate.
Your instructor may just be trying to enforce basics for the time being. So don't hold it against him.