In addition, in 2011 icons had some colors which helped to quickly identify what the item was. Now we don't have that and besides we see different icons for the same entity, given its ability to display custom icons based on origin.
http://technet.microsoft.com/en-us/library/dn660975.aspx

Besides it's not working that way (pressing F5 shows the 'old' case icon), it leads to confussion. A Case is a Case whichever its source was. It should be displayed consistently for users. If not, they get misconfused with other entities or simply don't understand why they look different in different parts of the system.