I commented on this question on Server Names for Content Porter: Could not connect to ImportExport service on SDL Tridion Content Manager server <IP address>
The actual error for entering "http://" in the Server Name for Content Porter was not really related to that question, so I'm posting the error and reason in this separate Q & (community) A.
I sometimes get the following in Content Porter when setting up an export:
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SDL Content Porter 2013 SP1
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System.Exception: Server name has invalid value.
Server name cannot be empty and must contain only alphanumeric characters.
at Tridion.ContentPorter.Presenters.SelectServerPresenter.CreateServerInfo(String protocol, String name, String port, String description)
at Tridion.ContentPorter.Presenters.SelectServerPresenter.SaveServer()
I added the reason below.