I came to Thailand about every other month for a couple years (working) before coming here for work full-time.
During that time, I met with our sales rep, looked at factories, met with customers, suppliers, contractors and lawyers. I finally leased an office and hired a driver and an office manager and started interviewing staff, all without a work permit. Every time I was asked by anyone why I was entering Thailand, or what I was doing in Thailand I answered truthfully, and I never had any problem.
During the over twenty years I worked here, I had countless foreign nationals visit the factory for any number of reasons, all working, generally for a week, but sometimes for several months at a time, none of them with work permits, always answering honestly, and no one ever having any issue.
For visitors, I think the no work permit issue is greatly exaggerated.