We say you need clean roms as some tools (thinking ndstool and the frontends DSLazy and DSbuff along with similar tools) change locations of things within the file which makes no real difference to the DS (the DS just looks up files on a table) but if we tell it to patch a given location and now a new file is there it will mess things up.
Trimming merely removes the junk and most patchers will not cry foul if they have to add data or simply replace it. Most hacks add something to the game and this extra stuff usually means extra space (which was filled with junk).
In the case of the DS what is trimmed is determined by the header of the cart, any good tool changes this if it does change things and all the current "safe" trimmers do this too. Old style trimmers will also still work. Hacks as aphirst mentioned tend to not add the junk back when they rebuild the rom so trimming is not necessary for a lot of them.
The end user however does not really need/want to know all this and clean roms make a good baseline (everyone should be able to find/make them) so we suggest people use them and hackers use them for their baseline just to avoid the hassle with people using all sorts of roms.
You may also be interested in:
http://gbatemp.net/index.php?showtopic=127411
Regarding one piece, I believe there is a Chinese translation but nothing I now of otherwise. If you go looking and find it post a copy back here and I will see about reverse engineering it.
Edit: gbatemp thread on the subject.
http://gbatemp.net/index.php?showtopic=103142&st=0
