If you have, say, 10 kg in your bc and you hand your gear up to the boat crew before boarding, as is the procedure on many boats, they will curse you under their breath as their backs strain. If you split it between the vest and a weightbelt, it will be more manageable for them and you. On some bcs, a lot of weight might strain the velcro, although I think bc design has moved to more secure fasteners. On my ScubaPro Classic Plus, the weight pockets were pretty easy to ditch, but a belt would be easier.
Of course, you can remove the weights and hand them up to the crew prior to taking off the jacket. However, having said that, I've found when you do that, you get a lot of blank looks and it's so easy for one of those weight pockets to get kicked overboard.
I've dived with both integrated weights and both soft and hard weight belts. As has been mentioned, this is mostly a preference.
I remember once, before I owned gear, I had a rental plastic weight belt buckle break. I managed to recover the weight belt and when I returned to the store, I promptly bought an integrated BCD, which I could not wait to use.
I dived with that BCD for a few years, but eventually I began to dislike the integrated weights, and this is why: in cold water, there is a lot of weight in those pockets. It is difficult to distribute, you really have very little choice where it is, as opposed to a belt or harness which allow for positioning the weight for trim. (I don't use ankle weights because my legs are negative, I don't like a weight on my cam straps because it is very difficult to ditch.)
I stuffed as much weight into the back pockets of the BCD (non-ditchable) but still had what seemed like tonnage in the front pockets. SO that presents a problem, especially on a boat. Do I put the weight pockets into the BCD prior to donning it, making it very difficult to get on? Or do I wait, leave the weight out and then struggle to mash the heavy weight holders into the pockets.
This may not sound like a big deal, but in a dry suit, with 35 or 40 pounds stuffed into those pockets, it's very difficult to get the jacket on with the weight in, and very difficult to get the holders in after the jacket is on.
I eventually concluded the negatives of integrated BCDs are, lack of trip, some difficult of use in certain conditions, and occasionally not being able to find the right kind of weight for the pockets (ie. soft), oh, and, the problem of handing the BCD back up onto the boat if that is necessary.
Eventually I could not wait to get back to a weight belt, which is what I did when I switched to a BP/W for most of my diving. I can adjust the belt, not only prior to the dive, but on the dive for trim depending on the conditions. Belts are easier to hand up and if you drop it, it's easier to find that a little weight pocket.
My trim is better with a belt and I can distribute it where I want it. Oddly, I actually prefer hard weights to soft weights. The one real drawback of a traditional belt is the inconvenience of changing weights every time you change your gear configuration or switch from salt to fresh water, or vice versa. (Incidentally, I find it easier to change the plate back and forth between steel and aluminum.)
No piece of gear is perfect for everyone, but for me, I will take a traditional belt with hard weights anyday, just make sure it is a steel buckle, not plastic (I always carry a couple of steel buckles with me when I travel so I can make sure that's what's on the belt I am using).
Jeff