you know this is actually an excellent point, because so far in black and white, i find that i've only used the bike once so far and that was to cross that really large bridge.
otherwise, i find it to be a very pointless item at the moment. my reasoning is because there has been an emphasis put on your character stepping in very particular spots.
in diamond, pearl, and platinum, we had the pokeradar which we used to step in particular spots for the prospect of "rare" pokemon.
in black and white, we have the rustling grass, or the dust clouds, or even the flying shadows, that requires us to step in a particular spot for the prospect of a rare pokemon or even an item. if you happen to enter an encounter before you reach that spot, then your chance has been lost until the next appearance.
also in the desert area, there are "safe zones" where wild pokemon do not tread. so while you are training, you are usually very mindful of where the safe zone is in case you need to make a hasty retreat.
i think the days are gone where you could just randomly ride your bike in circles until either your eggs hatches or you enter an encounter. i think the games are making us more mindful of our steps which is also why i believe that there has been an influx of pokemon healers in pokemon black and white if noone else had noticed that.
i find myself thinking more thoughts like "how far can i make it into this forest before i have to tend to their injuries".
and to the people who say the bike is better because you don't need to hold the b button down...personally, i have never even noticed that i'm holding the b button down half the time. the same way i don't notice that i still am presing a+b and down everytime i throw a pokeball.
don't get me wrong, i like the bike, but as this topic has pointed out, i do believe the item has become rather obsolete.
also on a side note, the bike is actually the reason i used my masterball on rayquaza... because i thought it was such a pain in the rear trying to get to the top of the tower, that once i got to the top, i made sure i was never making that journey again. so the bike can be a bit annoying as well.