First, I can think of a couple casinos that cut maybe not 4 decks but better than 3 from a 6 deck shoe. I'll name one...Longhorn on Boulder Highway. Tuscany is pretty close to 3 decks cut. and while South point is usually about 2 decks cut (67-70% penetration), I have seen them cutting 3 decks when they get rattled (after a visit from a team of counters). None of these are the 4 decks cut that you mentioned, but none of these examples makes any sense for the casino either. What is that old saying....cutting off your nose to spite your face?
So anyway, like you said, every game has unseen cards. I personally won't play anything less than 75% penetration unless there is something special drawing me to that game. But at 75% pen, I count through the shoe, get a good high count, there is no guarantee those high cards will even come into play. I mean at 75%, there is 78 cards not in play.
So, lets say I walk by a game that has 2 decks already played. As you correctly said, that adds to unseen cards, basically making that a game that has 3.5 decks cut instead of 1.5 that I usually seek. I would never consider playing that game off the top (after the shuffle at start of play) and the reason is that to get a significant high TC that turns the game to +EV, (let alone max bet TC, which is what we are really after), becomes much less likely. Most likely you are just going to play through a bunch of negative/neutral at best counts with such shallow penetration.
BUT, the difference is, that we are not playing this game off the top, starting with a zero count. If I walk by and see a bunch of small cards on the felt, resulting in a running count of lets say +8, +9, that is already a TC of +1.5. The game is already +EV, even with that increase in unseen cards. It's already +EV and the chances that it gets to a much stronger count are greatly improved, because we are not starting at 0. We have already cleared the first hurdle. We are already to +EV. So yeah, that game is worth jumping in and playing and especially if there is only one or two players.
So yeah you are right, I would never consider playing a 50%-ish penetration game, but you start me off with a significant plus count that changes everything.
As a primary style of play, I don't think much of a pure wong-in. A player will spend all his time searching and back-counting and get very little actual play. And when he does jump in after standing and back-counting, it's a pretty easy read for anyone watching. But as another tool in the toolbox, just happen to be walking by a table and pickup a count instantaneously that creates a playable opportunity....that is what it is for me.