Secondary Index + client field

I posted this already as an answer to another thema but since it is very important to me (and since there were some mistakes in my answer) now I make a new theme out of it:

When I look at non-unique secondary indizes in the data dictionary (Release 4.6c) I find that around two of three non-unique secondary Indizes of the standard SAP tables (I looked up by checking the tables dd03l, dd12l and dd17s) have the client field (rollname = mandt) as an entry, one third don't have the client field at all.

I wonder what could be the arguments using the client field in a secondary index or not.

Since we have performance problems in using huge data tables it would be very important for me to know if the usage of the client field as an index field brings more advantages or more disadvantages. We don't have any transactions where the "client specified" clause has to be used.

Kind regards Axel Kiltz