First off, always have a 2nd controller when company want to "throwdown" 
.
That second Turbo Pad must be pretty lonely without a Tap.
Second, all the other consoles were built with the idea of co-op play, so one item was need to play with a friend while two were necessary to fill that role for the Turbo Grafx. The Tap did allow up to 5 players but how many games allowed up to 5? I know the Bomberman games did.
I mentioned the 5 player aspect just because one plus about the Tap is that extra player over 4 player adapters on other consoles. On the TG-16, the Tap is well worth purchasing just for Bomberman alone, but Bomberman '93, Dungeon Explorer and Dungeon Explorer II are all top quality games that genuinely benefit from 3 or more player simultaneous play. Motoroader is also lots of fun and since people love Smash Bros games so much, I'm assuming that Battle Royale would be entertaining with 5 human players. I'm not sure how many of the sports games allowed for up to 5 players since I've never been a fan of the genre.
Guitar Hero and Rock Band came with the necessary tools to play upon purchase like how the Light Gun was used for Duck Hunt but I don't really see the fascination in pretending to play music except provide entertainment for group gathering. I didn't like the idea in having to buy accessories providing features that were already commonplace in consoles (i.e. 2-player co-op, A/V output). And comparing old-gen prices to new-gen isn't sound water under the bridge unless we're talking about the retail price for a Turbo Tap from 1990. 
Guitar Hero and Rock Band sell as a lone game, game + guitar or game + band set... but they all cost more with the instruments. They could be designed with optional modes that are pad-friendly, just as most Wii games should give you the option of using a Gamecube pad, but companies aren't in the business of giving game players what they want.
2+ player games are better supported on the TG-16 than light gun games are on the NES, but I only ever saw the light gun sold separate from any games or bundled with some NES packages, which always cost more than the base set.
Things may have changed, but the PS3 originally shipped well into the hi-def generation with only a composite cable and the Wii doesn't do any of it's limited online abilities without purchasing accessories, so it's still commonplace for consoles to ship without commonplace console features out of the box.
I didn't compare gen-prices, I said that as an unemployed kid back in the day (1990'ish), the Turbo Tap was easily affordable. A few years later I even bought a Duo Tap and extra Duo Pads just for the hell of it even though I had the Turbo-to-Duo adapter. Games back then cost the same here as current ones do today. The Tap and Pads each cost just under half the price of a game. Proportionately, a second Wii remote + nunchuk or a wireless Xbox 360 pad cost about the same as a Turbo Pad + Turbo Tap did.
Anyways, as you can see many other people outside of Japan didn't mind the single controller port.
One cost-saving feature of TG-16 games is that they fit in CD racks, so you didn't have to shell out for
another fancy dedicated game rack.
