They usually have a lot of replacements during the summer...the "YAWN" season for sports. No championships or playoffs (after the NBA and NHL) and the MLB Regular season is sooooooooooooooooo long and boring that there is little news during the end of June until August to really report, so often time Mike and Mike take their vacations during this time. When college football and the NFL start up, they roll every day and every week without break.
If you were going to allow your best morning team to take a break, when would you allow it? I think they make the right choice. I rarely listen during the summer either, but once the holy trinity of sports starts (College Football, NFL Football and College Basketball) then I listen every single day.
Rob