I'd argue that you could probably just look up your calendar in whatever system your assistant uses.
Shared electronic calendar software has been around for a while now, and there's only so many hours in the day...
What am I missing that means you can't personally schedule your own time with a customer or supplier ?
Isn't it just a case of opening your calendar app and finding a mutually agreeable half hour in there somewhere ?
EDIT: There is a way to accomplish the same end result without triggering my spidey-sense, though... it's for the too-busy executive (such as yourself) to not be the one to contact me. Have a more-available subordinate be my primary contact from the get-go. They can then arrange a meeting with the executive as a sort of side benefit. That way, the person that I have the relationship with is the one who has time for that interaction, and I won't be getting "pawned off".
I would rather talk to the person who knows and controls your calendar. They are the true source of power and they are the one who can actually get things accomplished.