I've always assumed that phrase meant you had markers out for the entire line at some point.
They only care about if you take out a marker and then only play with a small portion of it and take the money elsewhere. They expect you to use the money you take out at their casino and then to pay it off if you win. Those are the only rules. Size of your credit line or how much you take in markers against the credit line have no bearing on your standing with the casino...only how much you are betting.
Second. The only time they may look to change your credit line is when your license expires or your credit line has sat unused for over a year. When you go back to re-activate the credit line they may look to see that the bank accounts tied to it have around enough money to cover new markers taken on the account. But to address the OP: They never lower your credit line because you haven't taken out enough money. They gave you that credit line because your bank accounts showed you can handle that much debt if you lose. They figure sooner or later you're going to have a really bad trip and they'll get the $10,000 out of you...or as close as you're willing to go.
But the point is if you have a large credit line and play low, you aren't playing up to your credit line as you could have played the same with a much lower credit line.
I also don't think playing to your credit line has anything to do with whether you ever have your full credit line out at one time or not. There are trips when I may take out one marker and get on a hot streak and never have to take out another marker on that trip, but my bet size (and Theo) would still indicate that I am playing to my credit line, or higher.
Yeah I get your point and mostly agree. It's more about not making a fake show of setting up a big line or front money to make an impression and then not playing big.