You haven't been using it long enough so. There was a point in time were svn supported using merge but failed to record any information about what had been merged to where. So if your repeated the operation, you got a giant clusterphuck of a result. Same thing applied to subsequent merges, since there was no information stored in subversion you had to know beforehand which revisions were previously merged.
The consequence of this was that many teams ended up having to create custom tools to perform the merge and record the information somewhere else, or require that developers recorded which revisions were merged in the commit message.
The subversion developers eventually realised their mistake and added internal metadata to handle, so that if you merged r100 of branchA to branchB, and then subsequently tried to merge r200 of branchA to branchB, subversion would realise that everything prior to, and including r100 on branchA was already merged, and it need only merge the differences between r100 and r200.
Oddly enough though, by then most devs had already abandoned it for just about anything else that had sane merging.