Once in a while, threads with similar topics are merged, i think its a good thing, in order to stop unnecessary spamming and cluttering of forum space.
But, when threads are merged, it should be taken care of that the direction of discussion should be maintained. And the degree of similarity in topic should be discussed before merging.
For example:
"Fav movie" and "movie recently seen" threads are merged. I as an old member know, that they are one and same thread. But that may not be case for newer members.
So while I was searching for some info on similar thread, I found it very cumbersome and confusing.
The other disadvantage is someone will pop a similar naming thread once more. And then again the same cluttering problem. Even worse is someone asking you the info you have provided already.
So whenever threads are merged, the topic should be changed to indicate both topics clearly, or if possible the other thread may just be link to same thread(i am not sure such feature exists.).