Physics of downward firing woofers

The sound waves do indeed, at least to some extent bounce back from the floor, go through the driver cone and back into the enclosure and, while that's not necessarily the greatest strategy sound-wise, it will not really mess things up all that noticeably if there is sufficient airflow at volume. Were all the walls of the enclosure to extend down to the floor cutting off the woofer to the outside air then at volume such a sub would sound absolutely terrible and probably blow itself up. But sound waves at low frequencies tend to be more omni-directional, so having them pointed directly at the ear is not thought of as critical when crossed over low enough in frequency. 

While not treating the woofers like higher range drivers and pointing them at the listener is not as critical, doing so, for a number of reasons technically, is still a good idea and all else being equal the direct setup will sound better in terms of things like phase response, frequency response...even dynamics.

This was likely a design trend that was originally employed to offer better integration into a decor by giving it a more "furniture" look, sometimes in conjunction with a reduced size enclosure for a smaller footprint. Otherwise in general there is nothing particularly good performance-wise about that kind of approach, but they are not really egregiously bad sounding or anything.

Maybe the only real listening-oriented problem I could say that I’ve ever really run across in downward-driver designs is that, as a rule, I find there can be a big difference really between "bass impact" (or "wallop") and "bass dynamics". It’s possible to have lots of impact and yet still have (sometimes) even dreadful bass dynamics. Yet many people might be inclined to say that that kind of sound is as good or even preferable to one that tics all the traditional boxes in bass reproduction (???). Although it’s usually not to quite that objectionable degree audibly, that to me may be about the worst thing you can find yourself getting into with downward designs, that and the slight loss of timing that seems to go with it. But, it’s possible these designs may work as good or better in some "problem rooms" where there may not be enough space to properly present a direct setup. But, that’s one reason why I say above that dynamics can be better with a direct setup. They are not necessarily any stronger, they are just presented more fully intact.

I know when trying to assess downward subs that there tend to be sooo many other variables involved (placement, room size, etc) that establishing the exact audible effect the downward arrangement actually has can end up getting pretty intractable.