But it (supposedly) not last a long as grout. However, in your case, it does sound like flexiblity is more of a concern, so dig out the old grout and get out the caulking gun.
PS: To get a REALLY nice caulking edge, place some type of tape (like masking or painter's tape) along each side of the joint to be caulked. After you've put the caulking into the joint, use a finger with a TINY amount of dish soap on it to smooth the caulking. Then pull up the tape (right away) and you will have a nice crisp edge to the caulked joint that looks professional, instead of a smeary mess.