@COWMAKAZE:
If you had a cat that died in a freak woodchipper accident, would you search for the cat's legs, tail, and other assorted body parts to duct-tape to your new cat? And to those that think the ship will be "Going Merry 2" or somesuch, would you name that new cat "Fluffy 2" if your first cat's name was Fluffy? Both of those are kind of morbid, wouldn;t you say?
Now replace "cat" with "friend" and you basically get what the SHs are feeling - remember from the Klabautermann that the Going Merry was a living being. The Going Merry is DEAD, and the attaching of its parts to the new ship will not help get over this fact.
. . . ew. Point well argued. But I do think there will be some sort of "ghost" of the Merry hanging around in some form or another - maybe not pieces of the ship or anything like that, but I just feel like Oda isn't 100% done with Going Merry - certainly he is 99.9999% percent done, but I just have an odd feeling that something else is going to come up. Creating the new ship using some aspect of the old ship seemed like a reasonable way for that to happen.
For the record, I do believe the physical ship itself is destroyed, but individual pieces, its memory, and a few intangible aspects are not necessarily dead. Especially considering Usopp is still in the midst of his identity crisis. It seems to me that part of Usopp's "getting over the identity crisis" thing would involve some kind of proper closure for the destruction of the Going Merry.
Perhaps using GM pieces in the new ship wouldn't be the best solution. But if Fluffy died in that horrific accident, wouldn't you want some kind of closure to the situation? A funeral, cremation, ect?
Right now, I would say Usopp is going through the grieving process, which can usually be broken down into five steps:
http://www.cancersurvivors.org/Coping/end%20term/stages.htm
Usopp has already admitted he was in denial over the Going Merry's condition during his second encounter with Franky (prior to their capture by the CP9). That's the first stage. Anger, the second stage, would be his fight with Luffy. Stage three, bargaining, was well-represented by his attempt to fix the ship. By the time Sanji rescued him and Franky on the Puffing Tom, he was somewhere in the midst of the depression stage.
Right now, Usopp is working towards the final stage, which is acceptance. That will most likely occur at the end of this arc alongside some form of formal closure for the ship, even if it's just hanging of a picture of the old ship in somewhere in the new ship.
Yes, there are other factors involved, such as the paralell to weakness, but I don't think anyone can completely throw out the fact that the Going Merry was a gift from Kaya as one reason why Usopp is upset. Are there others? Yeah, probably.