Years ago I had a TG16, the lack of affordable games and CD hardware was a massive turnoff. At the time I stuck with TG16-centric communities and I often heard many complaints about it, like its needlessly large size, dumb back cover and the single controller port. Some said why not 2 controller ports? That never made sense to me since the system was always designed to use 1 controller port, then you add a multitap to get all 5 controllers.
To that end, I ask the PCEFX community here, should the oversized TG16 have had a multitap built in? If it had the original mini DIN connectors, there could easily be room for 5 controller ports on the front. Sure, it would have been more expensive, but imagine the marketing effort that would have been possible.
Because of how the system was made, and how the TAP was designed, simply adding another port on the system would have really complicated things. And that would have meant incompatibility between JP and US products, most likely.
The TAP itself is only made of a single GAL chip - not much logic. But I doubt they would have ate the cost and included it with every system, but they could've had possibly had a deluxe system package with two controllers, a tap, and a better packin game (or two). I dunno. Or maybe include a really cheap 2 port version of the tap with all systems? Japan had 3 port taps.
Besides, look at their business model: want to upgrade the system? By these additional items; booster, booster+, 5 port TAP, arcade stick, CD rom, etc. They were taking the Japan business model and applying it to the US market.