"Although some exercises help condition and others speed improvement, there’s one all-important activity that assists both. That activity is sparring. There is no substitute for sparring. You must spar regularly and often to become a well-rounded scrapper, regardless of what other exercises you may take. Sparring not only improves your skill, but it also conditions your body for fighting by forcing your muscles to become accustomed to the violent, broken movements that distinguish fighting from any other activity." - Jack Dempsey
Plain and simple... to improve at your sport of choice you MUST train with uncooperative resistance on a VERY regular basis! Now days I have a different attitude for those of us in a "hobby" where we take massive ammounts of trauma to the brain. That said if you are boxing/kickboxing you need to work sparring. Not necessary to hit and get hit with 100% power, but you do need to receive and give strikes.
In regards to any grappling you should certainly be "rolling" at some level every class (which at NWJJA we do) with uncooperative partners.
Food for thought for those of you out there in the combat sports.