[Proj] puzzled with +towgs84=... being ignored

[J. Luis]

Hi,

I'm very lost with this results where setting the +towgs84 is ignored 
unless I set "+datum=WGS84"
Why so? and why is the source "+ellps=WGS84" ignored as well?

cs2cs +proj=latlong +to +proj=sterea +lat_0=52.15616055555555 
+lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 
+ellps=bessel 
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812 
+units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

cs2cs +proj=latlong +to +proj=sterea +lat_0=52.15616055555555 
+lon_0=5.38763888888889 +k=0.9999079 +x_0=155000 +y_0=463000 
+ellps=bessel +units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

********** Now explicitly set the source ellipsoid (but nothing changes) 
**********
cs2cs +proj=latlong +ellps=WGS84 +to +proj=sterea 
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 
+x_0=155000 +y_0=463000 +ellps=bessel 
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812 
+units=m +no_defs
4.5 62
108156.39       1561986.31 0.00

That is in ALL above cases the 'towgs84' seams to have been ignored. 
However, if I use "+datum=WGS84" than the result changes

cs2cs +proj=latlong +datum=WGS84 +to +proj=sterea 
+lat_0=52.15616055555555 +lon_0=5.38763888888889 +k=0.9999079 
+x_0=155000 +y_0=463000 +ellps=bessel 
+towgs84=593.0297,26.0038,478.7534,0.40685733032239757,-0.3507326765425626,1.8703473836067959,4.0812 
+units=m +no_defs
4.5 62
108163.14       1562220.06 -39.17

Joaquim