A general example would be given the string "foo bar baz", your solution could find "foo%" or "%baz" efficiently, but not "%bar%". Its not out of the question if what you really want is a suffix search, but the general problem of finding an internal substring is still less than optimal.
Firstly, there are two important points to consider. Currently, BigCouch is more or less a superset of Apache CouchDB. The only patches we have to CouchDB sources can and should be back-ported but require that we solve a couple possibilities for bugs in non-clustered deployments. Secondly, Erlang is a language which allows for an easy mish-mashing of code so that once we have back-ported these patches there's no real requirement for a merge at all.
There are also a few things that we're discussing in the CouchDB community that could very well contribute to not needing to merge the projects. Specifically rearranging our source tree to be more prototypically Erlang as well as some tools like a couch-config script that could allow plugin-type extensions to CouchDB.
In the end, its hard to tell how things will shape up. It could be a full on back-port, or it could just be a general improvement to CouchDB's source tree and build system so that BigCouch is strictly "CouchDB + Other Erlang Apps" if that makes sense. And with my CouchDB committer hat on, it really depends on what the community wants. Its easy to fall into think of the trap of "it's obvious" but we also have to consider that others are taking CouchDB and porting it to mobile phones. What we end up with in "core" CouchDB has to consider a lot of use cases.
BTW, I have only set up BigCouch a couple of times to play with it but it is very impressive.