Or am I missing something here?
Suppose there's a sink that washes and dries, and there are two people who want to use it. The second person will spend 30 seconds waiting, and 30 seconds washing/drying. So, 60 sec total.
If there's a sink and a separate dryer, the second person will spend 15 sec waiting, 15 sec washing, and 15 sec drying. So, 45 sec total. The second person gets a 15 seconds savings, because he can start washing as soon as the first person finishes washing, rather than having to "block" until the first person is done washing and drying.
[1] http://en.wikipedia.org/wiki/Pipeline_(computing)#Concept_an...
1 sink and 1 dryer: first person leaves after 30s, after 60s 3 people have been served.
2 sink and dryer combos: 2 people leaves after 30s, after 60s 4 people have been served.
I think the analog is that parallel processing is better than pipelining.
Because typical bathrooms have 4 times as many sinks as they have hand driers, and I still see sink lines more often than drier lines.
In general (at least in the US) drying takes far longer than washing. Because most people wash quickly/poorly whereas driers are on timers and most are horrible.
And when your hands are still wet even after negotiating the unresponsive sensor and waiting an entire drying duration, it's understandable that many people would just as soon skip it.