@PlayForFun points out that it’s easier to keep an extra versus feeding him away since you probably don’t have a shot at getting him again until next December (there are a few exceptions).
In most events, I don’t see the need to have two on one team. He does decent damage, but I have plenty of other slow reds that can fill another spot.
So where are we hero limited? Mythic Titans, W3K, Towers, Tavern of Legends, Contest of Elements.
I don’t think ToL is worth investing in a second Buster. Many can already easily complete. Others, he probably isn’t the difference maker.
Contest of Elements and Towers could make sense, but I worry about a three star survivability against harder foes. I could see him being useful in earlier levels of those, and with an extra one you don’t have to worry about losing that one.
W3K, he may just be depth, but I don’t see myself using a slow 3 star, even for cleans.
Mythic Titan is one area I could see him being useful. A dup would at least get used against the green MT for three or four attacks. I’m not sure I need two for the other colors. I generally do one red against against the red, dark, and holy MT. Maybe a dup could help with my strongest attack? But I could also just not use that dup in my red team.
I generally keep some unuseful heroes on my roster because you never know, but I haven’t felt any need to keep a second buster.