The call letters for 105.9 don't matter much, they are only mentioned at about 3 minutes between the 55 biz and 58 traffic/weather.
They are one line in the ratings now, meaning for ratings purposes it is counted as WBBM AM.
They are now saying "WBBM newsradio" to id most of the time. At times they say "WBBM newsradio 780 and 105.9" and still use "780 and now 105.9", the last one seems to be used a lot less lately. I can usually pick up the AM at night where I am.
If they did not simulcast 100% of programming, they would have to have seperate line ratings, so then it would probably be more of an issue.
The call letters really mean little to younger listeners, who identify with the handle and frequency.
The ad community knows WBBM FM is B 96, to change those calls could cause advertiser confusion. Something they don't want to happen.
It might not be a bad idea to change to something close to WBBM for 105.9 say WBMM, WBBN, if they are available, it is not a neccasity however.
CBS may want to keep the WCFS calls parked for future use on another station.
WSB in Atlanta has beem simulcasting for a while on FM now, they have the FM calls now as WSBB, they have a WSB FM which is heritage station in the market that they did not want to mess around with either.
I noticed lately they have been naming the FM before the AM on air. The website also places the FM ahead of the AM. Perhaps WBBM will do that in the future.