can someone explain to me what are the advantages of a deck stepped rig over a hog stepped one.

I believe the main advantage is that the mast can be raked through a greater angle with the deck stepped rig, without having to change the spreader angle to maintain the amount of mast prebend.

Since a deck-stepped mast pivots around a point the same height as the shrouds, it does not change the spreader geometry as you rake back. This means you can rake further and depower very effectively.

There is a little more text on masts at this link:

OK, silly question then, why not take this further and step the mast at the gooseneck? Then you can have rake independent of mast bend - cherubs & moths have had this type of arrangement for years. The bottom "stub" mast can be locked off with lowers and puller. It ensures that all the high load gubbins - gooseneck, lowers, etc. are all on the fixed portion = made stronger and the light floppy bit is acting in pure compression from the shrouds. Or am I just mad?

I'm sure 3400 Dangerbat did somthing like this some time ago James M-S might be able elaborate on this. Lots of strings and pullys if I remener from the pictures.