More efficient repairs. Instant mapping of many files as one. It doesn't need to maintain an index of its one. This is much faster and will save a lot of disk space. Go around the file size limit for the operating system.
Key reads are slower.
If you then do a 'read-next' then the merge table handler will need to search the read buffers to find the next key. Only when one key buffer is used up, the handler will need to read the next key block. This is only a sugestion.