Yes, I'm curious as to why the boom sleeve needs to be manufacturer suppled, as well. Is there some reason, other than to line the manufacturers pockets, that they should supply it?
Yes, we are beholden to the builders. But look at what we get for that: a boat that is virtually the same (with only minor building tolerance type differences) throughout the entire world. So, I can get on an airplane and fly to anywhere in the world (and they sail Lasers on at least 6 of the 7 continents), get in a Laser and it will behave exactly like my boat at home (and I'll finish in exactly the same place no doubt!).
In addition, we get a boat where we have virtually no measurement process at any of our major regattas. "Measurement" in the Laser Class, at the most stringent level (the world championships) consists of "inspection" where the measurer looks to make sure you have builder supplied parts and that you haven't come up with some crazy new rigging system (which, in the end, is really not going to make you faster - except maybe carbon tillers since they don't flex). In North America, at most events we "measure" sails which really means checking the sail button to make sure it is "legal" (i.e. a Laser sail) and then checking that the correct numbers are on it (and, believe it or not, the sail number checking is for the competitor... if the race committee can't figure out your sail numbers when you cross the line then there is a good chance you are not going to get your correct finish position - I've been on enough finish lines to see this in action!).
Compare this process to other one design classes where measurement can be a time consuming and painful process. For example, I sailed the 2005 Etchells Worlds held here in San Francisco and we budgeted an entire day for the measurement process - this after the hull had already been checked! As far as I'm concerned, that's one less day sailing!
How does this come back to the boom sleeve? If left open then we have to write a rule that specifies what a legal boom sleeve is AND then specify a procedure for measuring it at regattas. If we want to stay true to the strict one design standard that has made the Laser what it is today then we are better off with this being a builder supplied part.