It would sort of ruin the game for me, I think. One of the things I like about Bohnanza is that superficially it's a pretty easy game: certainly you can bring it out with non-gamery types or children and it will be a hit, due to the colourful cards and the social aspect of the trading.
However, there are strategic undercurrents to Bohnanza that set it apart from, I don't know, Uno and the like. And one of the main ones is the fact that, if you're paying attention, you can realize that, as the game progresses, the value of beans is changing as cards leave play to become gold coins. If you are paying more attention than other players, you can be planting the most advantageous beans remaining while they carelessly plant beans most of whose brethren are sitting in people's coin piles.
If you remove the facedown cards rule, you still have a fun game, but I think it's a game that's almost totally ruled by charisma. If you're the sort of person that people like trading with, you'll probably win. (The ladies in our group are less cutthroat than the guys, and quite often do very well at Bohnanza, simply because we are obviously checking values of beans and saying "no, no, that's not a very good trade for me, I'd need another card" - so they stop trading with us!) So it makes me much happier that there's an element to the game which allows good sense of which beans are in and out of the deck at any given point to make a profit. I like that having some smarts can allow you to maximise your chances against the luck of the draw.
Having said that, Bohnanza is above all a supremely fun game, and when I sit down to it I quite often don't bother to obsessively card count, or go into "analysis paralysis" over my moves. You could change the money-representing mechanism and I'm sure I'd still enjoy Bohnanza about the same: I'd be sad that the potential for some cunning and counting had disappeared, on some level, too though.
One final thing that you want to watch for is that the game will last a lot longer if cards aren't being removed from the deck in each pass. I really like the fact that the first pass is a pretty leisurely affair, while by the third pass people are saying "holy moley, is the game that close to finishing?" It adds a lot of tension to proceedings. In fact, the ability to cash in or to hold up from cashing is a way that players have to control the pace of the game and the constituency of the deck: that's another way that changing the rules would adversely effect players' chances of being "clever" in the game!
In short, I think Bohnanza is a spectacularly well-designed game with a lot of depths that you might not notice on your first few games - as you'd expect from the great Uwe Rosenberg. I personally wouldn't tinker with it, I think it would lose a lot. But if you want a longer, more carefree, more socially-oriented game then it could be worth a try anyway!
