WRT to europe & JAA licences...
Unless it is an instructional sortie and the safety pilot is indeed the instructor then the safety pilot is only able to log SNY.
Unless they were the sole manipulator of the controls they cannot log ANY take-offs or landings. But logging these as an SNY is likely to raise eyebrows. In the UK I recommend nobody goes down this route.
The SNY time cannot count towards ANY flight time experience totals - INLCUDING total time. This means that SNY flying cannot count towards XC, nor instrument flight, nor IFR time. Absolutely nothing may be counted.
Frankly logging SNY I feel devalues the log book as its almost plane-spotter level logging.
This is a familiar question to me as an instructor - particular from hour building pilots desperate to gain hours. Ive heard the arguments about 'required crew' many times over. My basic advice is simple. Earn the money. Buy the flying. There is no short-cut to experience.
WRT C152/172 issue. Interesting because over here these are in the same class and therefore this issue does not occur. If you cannot teach on C152 - which means you should NOT be flying with a student. You cannot claim any hours as you are SNY.
Hours as safety pilot count for nothing.
At least thats the position in europe.