SMTPに限らず、意外といろんなことに当てはまることなのでしょうね。
SMTP's strength comes primarily from its simplicity. Experience with
many protocols has shown that protocols with few options tend towards
ubiquity, whereas protocols with many options tend towards obscurity.Each and every extension, regardless of its benefits, must be
carefully scrutinized with respect to its implementation, deployment,
and interoperability costs. In many cases, the cost of extending the
SMTP service will likely outweigh the benefit.Request for Comments: 2821 2.2.1. Background (p.8)
