I can think of a few possible reasons you may want to develop your own new board from scratch, instead of using YaBB SE:
1) You want a whole slew of new features
2) You want to build a board using a completely different underlying structure
3) You want to own the board and/or use a non-open source license
4) You want the challenge of building a new board from scratch, or...
5) You hate YaBB SE.
To which I say:
1) Thats what modding is for. Pointless to build yourself a new board when you can save time and modify an existing one.
2) A legitimate reason, although rumor has it YSE2 is going to be a big step on the path to a drastically improved internal structure, and it sounds like 1.5 is a strong indicator of that
3) Again, understandable, if you want to go to the trouble, building your own board with a privately-held license gives you more control over what is done with it. But the Open Source movement is not only noble, its also very practical for ensuring the long-term life of the board.
4) Good luck, you've got a lot more patience and ambition than I have. I'd rather use an existing system and modify it to my needs.
5) Please don't take this the wrong way or too personally, because I mean it in the very kindest possible way: Get out!

If you don't like the software, what on earth are you doing here? That's what this forum is all about!