i have been using noka s60 phone sfor wa long time now and am currently using n70. earlier i have used 6670,7610,6600, 6630 and 6680.

I have noticed that in the bb5 phones once you open up contacts, call log or the camera application and even if you exit through the exit key still these applications keep runnng in the background and eating up RAM and thereby your battery drains faster ( i have checked that these applications keep running through appman and unreal device manager).

I had noticed similar sort of behaviour with nokia 6630 and 6680 as we3ll whereas this did not use to happen with the 7610, 6670 or the 6600??

Strangely if you use the menu key and then exit these applications through a lng press of the clear "c" key these applications close completely. Can anybody explain this??

Also what impact does it have on the phone

Well from what I've read it's purpouse is better performance so those apps will start faster.
It shouldn't have any impact on your phone since you have an N70 which means LOTS of ram, 32 mb for running apps! Which I believe is still in this day more than any other S60 phone.

but as i mentioned 6630 and 6680 have much lesser ram and the same problem remains with them as well