I think it is all fine. If you want to sell things as well as buy things it is easy to make two threads and point one to the other in each thread. This way people know what they are looking for in each section. In this case, one section is not littered with something you are not even looking for. If I was looking to buy something, going through hordes of WTB threads to find what I was looking for because the search can not filter such things out, can be a little annoying.
Also, if you leave the forums over such a small change, then you didn't seem to care about the site as it is. I do not see the logic in wanting it merged. Very few people just browse to just browse, if they are looking for something, they want to make it as quick as possible. Skimming over WTB threads when you are not interested in them at all is pointless since you do not have the interest. Before people post something for sale, they can easily check the WTB threads to see if someone is looking for it, if not they put it up for sale and perhaps someone will have interest.
The only argument on it not being a good thing, is traffic. Quite simply, you can only guess on that since it all depends on what you want and others having it. With WTB threads being in the for sale area, they would get pushed so many pages down quickly that there is nothing to say it would be much different than having its own section. The fact is, you just can not determine figures regarding offers, since no item or no point of time is going to be the same. If I wanted DDR2 memory this week, I could get more offers now than 2 months from now, or vice versa, no way of knowing.