Ofcourse there are several adnavtages of having unicode, but it completely depends upon the management decision whether they are ready to perform it. and also necessity of having UNICODE increases when organisation operates at several locations and supports several languages.
As you rightly said having UNICODE requires extensive testing on ABAP custom codes to make them unicode complaint. But going ahead while when you are integrating JAVA applications in landscape which are only UNICODE you would encounter problems, you would have to perform the upgrade later or otherwise continue with the conventioanl technology.
Unicode has become one of the most essential international standards in the business software world for code pages. All future SAP applications are Unicode enabled. We have noticed that non-Unicode systems face increasing restrictions and imply greater risks.