Hi,
I'm also using WORKAREA before since I'm used to it..
But many members here are ALWAYS suggesting to use field symbols instead for better performance..So I just ASSUME that field symbols should be used when doing BW routines ONLY..Because I asked several ABAPers and found out that they PREFER to use WORKAREA since field symbol is hard to DEBUG in some cases..
Hope it gave you an idea.. 
Regards,
Loed