If you have the datasource configured in the app-servlet.xml, it will only be available to applications that have access to that spring context.
If this one app is the only one that needs this datasource, that should be fine.
I use jboss/tomcat, so I'm not familiar with the configuration of just tomcat. I'm guessing that 'placing the database connection in $CATALINA_HOME/conf/server.xml' makes it available to any tomcat app. If that's the case, it would depend on whether you need that functionality. And in that case, I don't know how you'd make a pooled datasource generally available.