Here's a link to the vessel documentation center's website if you don't have it already, so you can double check everything I say, sense I don;t really know what I'm talking about...
USCG National Vessel Documentation Center, Home Page
We are about to get our boat documented so have been doing so research. Here is what I found....
The up side -
-It can make going into other foreign countries easier, even if they will accept a state registration. This has never been an issue with going to Canada, our only foreign port experience, and the Caribbean shouldn't be an issue either from what I here, but you won;t have to wonder if you get it documented, or if you want to go elsewhere at the last minute, or if policies change.
-If you move between states very often, you won't have to change the registration numbers. This is the big one for us at the moment, as we have moved the boat between Oregon and Washington a few times an intend to do so some more. Washington requires registration ofter 60 days (IIRC) so just taking the boat to the sound for the summer could require changing the numbers, then changing again when we get home. Of course, enforcement is another issue, but I don't like wondering if I'll get caught. So far I have changed the numbers twice, and plan to be documented before I need to do it again.
The Down Side -
-Selling a documented boat has a whole new set of rules, see the website for details. Just forms to file and what not.
-If the boat has a lien on it, there are more forms and rules, see the web site for details.
-You need to permanently affix the documentation number inside the hull, in a visible location, again, see the website for the details.
-I think you need to remember to re-file something every so often to let them know you still have the boat.
The Sideways Side (not up or down)
-You still need to register the boat in any state it is staying in for a while, pay the fees and get the sticker, you just don't have to apply / change the registration numbers any more. They still want their money for you using their waters and water related facilities (and rightly so) so you don't save anything there, just don't have to manage the numbers if you move around allot.
Well, that's all I know. Likely more then you asked for, and some you already know, but there you go anyway!