By using couch-DB you can have Erland I/O scalability and then use whatever you want for the rest of the server side logic.
If you have such a massive success that you need the rest of it also ported, then you can make this investment later on. The rest of us simply don't have the time to learn whatever is the fastest each month.
The fact that CouchDB is written in Erlang is only part of the equation.
You're not considering that CouchDB views are written in Javascript or that CouchDB uses HTTP as its connection protocol, which has more overhead than the typical binary protocol of traditional DBMSes.
Also, avg. response time in CouchDB is directly proportional to the number of view invalidations that occur. If your data is changing a lot your views will be invalidated frequently and for those requests avg response time will drop significantly. Throw in a reduce function and re-indexing can become an order of magnitude (or more) slower.
If you have such a massive success that you need the rest of it also ported, then you can make this investment later on. The rest of us simply don't have the time to learn whatever is the fastest each month.