Well it puts more stress on the engine, hull, floor, and stringers. If the weight is evenly distributed, it isn't that bad. But over time it will eventually weaken the boat.
No way..... I've weighted every boat I've owned and they're spotless when I'm done with them. You can abuse a boat whether weighted or not. Running around with 1200lbs of sacks all the time is just the same as if you had 10 people on the boat.
Well what is your definition of messing up your boat? You cannot tell me that adding 1500lbs of weight everytime you take the boat out wont wear down the boat over time. Sure it is the same as having 8 people in the boat, but 8 people will mess up your boat even more.
So the answer to 'will it mess up my boat?' is yes. But it is probably negligible wear. Now adding 3000-4000lbs plus people like some will definately hurt the transmission, engine, and hull over time.
My sacs caved in the floor behind the engine box in my old Prostar. Newer floors are reinforced to handle sacs on them. I think you should change your oil more than every 50 hours if you are running with a lot of weight. I change mine each 30h and it is already pretty black by that time. Compare the color of your boat's oil after 30+ hours running with 1500lbs+ to your car's after 3000mi. What is most important is that you maintain your boat regularly and use it a lot. A boat with 100+hours a season that gets regular oil and impeller changes will usually have fewer problems than a boat that is rarely used. Emphasis on "usually"
Yes, adding weight can “mess” up your boat in the long run, along with a many other things. Is it worth it? Heck yes it is! Just be sure to do the regular maintenance work and empty your ballast after you ride.