IT'S a MATTER OF DESIGN
HI LAPLACE ..A CPLD IS SOMETIMES WELL SUITED TO REPLACE LOW COUNT NUMBEROF GLUE LOGIC..
IF YOU USE A BIG CPU AN OTHER FUNTION CHIPS THE CPLD IS THE WAY TO GO, ESPECIALY IF YOU HAVE TO MEET IMPOSED TIMINGS AND POWER REQUIREMETS ..
IF YOU NEED TO IMPLEMENT A PARTICULAR FUNCTION THAT IS TO COMPLEX TO BE TACKLE BY A CPU .. OR THAT WILL TIGHT UP THE CPU FOREVER THEN THE SOLUTION IS A SPECILISED CORE .AND ASIC OR FPGA .THIS DAYS .THERE ARE MANY POWERFULL SOLUTION FOR HIGH SPEED EMBEDED LIKE THE XILINX VIRTEX II .BU THE CHIPS GO FOR $200
SOME EPLDS GO FOR $2.. AND A CONSTELATION OF SOLUTIONS IN BEWTEEN ..SO YOU SHOULD FIND WHAT IS BEST SUITED FOR YOUR PARTICULAR NEED ..SPEED ,POWER,COMPLEXITY.