IMO that would only matter to me if we also had stats on what we damaged and blew up, say I destroyed 30 hangers, 26 fuel and ammo, 16 radar, and 150 city buildings, then it might matter to me how many structures I assisted in their destruction.
The only benifit I can see from getting an assist message from attacking a structure would be I would know that I was hitting my targets and just not blowing them up. The way it is now, you hear your bomb and rockets explode but the structure still stands and you wonder if you hit it or missed it. Either a hit message or assist message would work for me.