A user ran into the issue that a job config had <mavenName>Maven 2.2.1</mavenName> yet there was only one Maven installation, Maven (latest). The job of course failed, but /job/*/configure offers no apparent fix: with only one installation, the dropdown version selector is hidden!
(The workaround is to add a dummy Maven installation, configure the job to use Maven (latest), then delete the dummy installation. Or to add Maven 2.2.1 and use it, if that was the intent to begin with.)
Would be clearer and better to just always show the pulldown. A little bit more screen real estate when there is only one installation, but still probably good to see what you are running so you are not surprised later. Definitely it has to be shown when there is no option corresponding to the current value!
Used the Maven installations and the native Maven job type as an example, but the same really applies to any UI to select a tool installation.
Also if there is no installation available, an empty pulldown emphasizes this. (There is already a warning in the case of the Maven selector.)