If you mean the studio version - they weren't. Reasons: according to Fred, they were outside London at the time (I think they were touring). According to John, the whole Band Aid thing wanted to involve newer acts rather than the oldies. Still Phil and David are in the studio version but I guess they were still "modern" due to David's chamaleonic character (always trendy) and Phil's solo career.
The live aid version, Freddie is there. Also Paul McCartney and at one point the two of them plus Bono sing at the same mic. Historical moment. I love the song but I think the way the "all-stars" team performed it that night was pants.