Producing products under the OGL without using the d20STL allows for one to ignore the restrictions that are inherent in the d20STL Guide. Some companies might be producing a product in which they wish to add character generation rules, or rules that show how to apply experience to a character, or even might contain artwork of an explicit nature, and thus would be a no-no under the d20STL.
There are a myriad of restrictions in the d20STL Guide, if one counts all of the terms that must be left untouched (such as how "AC" is defined, etc.).
Even the slightest infraction can result in the loss of the license, in perpetuity, and the requirement that all stock be destroyed, as well as requiring the company to track down all existing copies of any work produced and requesting that the current owner of that stock destroy those copies.
It could get quite expensive for an offending company.