I could understand the theory of having the water pump changed with every other or every cam belt kit if you were paying someone else for the labour. But if you're doing it yourself, why not change it if it's broke/leaking? I could find better things to spend £40 on. 
Because it may not last the next 40k, which means another stripdown, coolant all over your timing belt etc.
Preventative maintenence 
you could change loads of bits on a routine basis .... just in case!
:-/
I think a rubber belt can manage a soaking in coolant without harm. 
Indeed you could. I change my cambelt at 40k. It may go on to 100k. It may not. I change my oil at 3-4k, just as it starts turning from clear to brown, rather than the specified 10k. I change my plugs at 20k, even though they are rated to 40k, and will probably go much beyond, I change the air filter before its dirty, and the fuel filter regularly. I change the pads long before they are half worn at front (shame I didn't do the rears

).
OK, I had a cranksensor fail at an inconvenient time, but otherwise that MV6 has never, ever let me down.
I like to change service items on that car before they get to a point they are increasingly more likely to let me down, which may or may not be the reason it has excellent reliability - and remember, this car has a hard life.
Thats my reasoning for why I change things when I do, others have different priorities, views, opinions etc, which is absolutely fine

. Thats why I said that I personally change the pump every other cambelt
