Firstly as LARS controller, I don't want my radar display swamped with unnecessary SSR labels. We already have problems with label overlap and one only has to watch the gliders out of Lasham on their competitions this week to see how close together they are and how close the labels will be, rendering the SSR returns useless.
Secondly, has everyone seen the reports in todays papers about all road vehicles being fitted with tracking devices ostensibly for taxation purposes? Well it strikes me a Mode S transponder could be used by an unscrupulous government (if we ever had one) to monitor and thereby impose taxation on all airspace users. Course it could be argued that they are necessary for security reasons, but then you only have to read reports about people using false number plates to get round Red Ken's congestion charge to realise that someone somewhere will eventually invent a 'false identity' attachment for mode S transponders.