Compiling Java Code in BW 5 ( Maybe BW6 as well )

While wrtining Java Code or functions , why is it necessary to Compile within  IDE or at Design Time ?

Can't the compilation be deffered to Build TIme  or only when the  Developer wants to Debug ( runtime) within Designer/BS?  

Is'nt Compilation strictly/mainly a Build TIme actvity ?  If yes , why this departure when Tibco in general adheres to standards and guidelines very religiously ?