The real world response is actually pretty simple. There are very few slick tires available to GT drivers that meet all the needs. IE: If you want to use REALISTIC wheels. have you looked for 24mm realistic wheels lately ?? Almost non existant. The flipside is that there are alot of 26mm realistic wheels but NOT alot of 26mm slick tires available. The other aspect is related to the business. HPI, like many companies, are looking for ways to defray costs. Licensed products are one way to do that. When Falken shows interest, you pursue it. Guess what....Falken doesn't make slicks.
Bottom line is that there ARE legitimate reasons why slicks aren't used. As someone who also advocated using slicks I can offer my practiced opinion and also say that when I finally DID get them....they sucked on our un-prepared concrete track and I imagine they'd suck on our now asphalt parking lot track. They may be great for nice, smooth, prepared asphalt with traction compound and I'm sure they'd work on carpet BUT for good old natural parking lot tracks....I'll take the X Pattern Pro any day.