Are we talking bout the same merl? Not sure...
The (high) ankle strap prevents water flushing in when you crash, and it prevent the boot from rolling "up" inside out also - this is an ankle strap:
Whereas the instep/dorsum strap is something I hate when using straps and dont want, can not see its purpose really - like this:
I might not know the correct names for these straps, sorry
Peter