|From what I have gathered- Risk of getting dirt lodged into some of the small passages.
As mentioned, what seems to be good preventative maintenance is to drain the pan and refill once a year or so (depending upon mileage and conditions). This helps to keep the fluid refreshed, but the downside is that it only dilutes the old dirty fluid, without actually changing it.
On some cars, the return hose at the radiator/trans cooler can be detached and used to pump the fluid out-- I dont remember exactly how this is done but the car needs to be started up to do it. Not sure if it can be done on the Z32.