This particular broadband provider was nice enough to include the answer within their FAQ, and it is not at all what I expected [copied directly from their FAQ]:
FAQ: Should I leave my router switched on all the time?
Yes. Leaving your router on will make sure you get the best speed and performance from your service.
Don't switch it off at night! Regularly switching off your router can make it look like your service is disconnecting. If this happens, your broadband speed will be reduced because the exchange thinks your line is unstable and can't cope with higher speeds.
Aside from any technical reasons, I can think of a plethora of reasons why this might in fact be a bad idea:
Physical security is the best kind of security, preventing attackers getting their grubby mits anywhere near the bottom rung of the 7 layer OSI model. Physical security for a wireless, omnidirectional service can only be achieved through removing the power, stopping any would-be intruders in their tracks.
Leaving it on constantly however massively increases the attack area, which can be used to attempt to gain access. Once access is gained any kind of activities can take place, which could land the fee-paying subscriber in plenty of trouble such as; sending spam, stealing passwords, masquerading and cookie sniffing among others.
- Wasting power
Of course anything that has electricity running through it whilst not being used can be deemed a waste, hence minimising any such time is preferable.
- Fire risk
By leaving the router on all the time you are increasing the likelihood of a short occurring or other such malfunction which could be attributed as a fire risk.
There is also an active discussion: http://community.plus.net/forum/index.php/topic,80567.0.html/
Which, in addition to the other two points means I will be isolating my router for the foreseeable future and to be honest, who would want to risk those points for the sake of a possible yet temporary drop in speed?