Protector won't do anything to safeguard against problems like this, it isn't a magic forcefield!
I find that commercial customers are rarely even interested in protector anyway, it's an additional cost on top of the expense of the cleaning, they're looking for ways to reduce costs not spend more.
Most day-to-day marks and stains can be dealt with sufficiently with your normal cleaning and spotting techniques & products. Just imagine if you sell protector and then come back and can't get the stains out that they thought they were going to be preventing by spending the extra money. They're suddenly going to get the feeling that you've ripped them off and may dump you.
I think that the best approach with commercial customers is to try to steer them into a continuing carpet care program, that way you're in on a regular basis and can spot-clean each time you visit. It's an uphill struggle, most commercial customers seem to be stuck with the opinion that they should only call us when the carpets look disgusting.
At the moment I'm trying to sort my pricing to reflect this advice, pricing higher for heavy-duty extraction cleaning and lower for bonnet work. I'm definitely nowhere near happy with my commercial work yet, still much to do before I'm in control rather than at the mercy of their spending budgets!