Some reasons:
1/ I don't have time to re-test and fix the bugs.
2/ I don't know what he'll do with my rom.
3/ He doesn't report all with me and do anything he like without my permission.
In beta 1, I warned him many times but he defied it. He broke my trust.
...
You can think that I'm prissy but I have to control my hack, it will make me fix the bugs easily. A small change can make a big bug, especially with a long hack.
The text may be bad but I will not change anything more because I don't have time to do this. I'm tired with a busy life and this topic. So with me, it's Ok. :)
First, you should choose a house, tree... for one tile. If it looks fine, the you can insert them.
If you make a lot of them, you'll gain the experience.