Installing plugins without restart don't work in case if non-refreshable extension is activated. In my case I had https://wiki.jenkins-ci.org/display/JENKINS/Persona+Plugin installed, and received following exception:

hudson.util.IOException2: Failed to dynamically deploy this plugin
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1137)
at hudson.model.UpdateCenter$DownloadJob.run(UpdateCenter.java:955)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Caused by: hudson.util.IOException2: Failed to install build-name-setter-plugin plugin
at hudson.PluginManager.dynamicLoad(PluginManager.java:365)
at hudson.model.UpdateCenter$InstallationJob._run(UpdateCenter.java:1133)
... 7 more
Caused by: jenkins.ExtensionRefreshException: hudson.plugins.persona.xml.XmlPersonaFinder@1664d71 doesn't support refresh
at jenkins.model.Jenkins.refreshExtensions(Jenkins.java:1970)
at hudson.PluginManager.dynamicLoad(PluginManager.java:358)
... 8 more