The weight of the shaft, if it's the true weight & not because of brass joint insert, will be distributed according to taper. A 4.5oz. shaft with a 13mm tip & .850" joint is going to be heavy enough at the tip to noticeably increase deflection. It's also heavy enough to begin slowing the reflex of the shaft down after impact. Nothing is gospel & there are exceptions to every rule, but the above reasons have been my experience.
Keep in mind that this is my experience based on my taper with 13mm shafts. Other taper shapes & tip diameters can most definitely have their own rules to follow. What does not work well on my cues may work great on somebody else's. I play with all of my cues, and am constantly experimenting/evolving in my methods. I am a somewhat serious player & am interested in maximum performance and ease of use. I have found that cues with 15oz. butts & 4oz. shafts with my dimensions have the overall best harmonics & balanced power. Everything seems to flow, come together & work in unison. From a player's point of view, it just feels better & is easier to use, as well as seems to be most accurate.
With heavy shafts I have trouble keeping the cue at 19oz. without putting the balance point too far forward, which after long hours can begin to irritate the grip hand's wrist. Too much forward balance also makes speed control a bit trickier. Combine speed control issues with a sore wrist & high tip-end-mass (deflection) and you get a cue that is unpleasant to use after 3 continuous hours of playing.
Light shafts are the other end of the spectrum. Keeping 19oz with the correct balance is easy enough, but the density of the wood is too low to give the snappy, strong rigidity that I prefer. The fibers are too loose in lightweight wood to give that sharp impact. Instead the wood gives way under impact & absorbs the energy, which in turn feels "dead" or like a thud. But 4oz. seems to be around the ideal weight for my taper. This is dense enough wood to give the strength but without having to sacrifice accuracy due to high tip-end-mass that heavy shafts have.
That's probably more of an answer than you were looking for, but as well described as I can. It's all about me, my personal preferences based on my personal experiences. Nothing above is gospel for anybody but myself. Even still I am evolving. I build every cue for myself, and folks who have similar feelings buy them. Two years ago I would have claimed heavy shafts were the best because I believed it. But now, a few thousand hours of table time later & a more refined playing skill, I have little by little changed my thinking on many things. As such, my cues little by little evolve with me & my game. I'll take the ride as far as it goes
