So much bull in this thread.
Spain requires flight plans for VFR only for flights in controlled airspace. Look in their AIP, rather than propagate this disinformation yet again.
Airborne radar which can pick up other aircraft is massively expensive. Only some military planes have it.
The case for transponders in GA is to ensure that TCAS systems carried by commercial traffic (this is mandatory for anything of any size) can detect the GA aircraft, should it get too close. And 'getting too close' usually means somebody got lost and busted controlled airspace. Had there been no CAS busts I doubt we would be seeing this situation, but they continue because lots of GA traffic is unable to navigate.
A small piece of the case for transponders is that some commercial traffic also flies in Class G. One can moan about this but the obvious solution is to have more controlled airspace! Now, do you want more CAS, or mandatory transponders?
Now, to trigger somebody's TCAS one needs only Mode C; no need for Mode S. If Mode C had been made mandatory years ago, or the UK and Europe adopted US-style Mode C veils around major airports, the fuss would have died down long ago and GA would probably not be compelled to fit Mode S.
But, GA has successfully lobbied against transponders and when it finally lost the battle (the Mode S battle is now lost; it's just a matter of time before they are a practical must-have for VFR or IFR touring) Mode S was the technology of choice because it enables ATC to selectively interrogate traffic.
IMHO it is unnecessary to mandate Mode S for the sector of GA that carries Elementary Mode S (basically anything below 250kt TAS and below 5700kg; above these Enhanced Mode S is required) - Mode C would have been perfectly fine and would have delivered most of the benefit to ATC. But this is what we have and we are stuck with it.