Even if you've no plans to, be able to is a useful discipline and keeps your code clean. For years I worked on a Solaris project that we also maintained a build for that no-one used on NT. But when a customer wanted an AIX version, it took as long to produce one as it did to find an AIX box to compile it on.

Same is true of databases. There are things that people don't even think about if they've only ever used one database. What's your DB's lock escalation strategy? Are cursors cheap or expensive? What kinds of indexes don't you have, and could any of them ever be useful? And so on...