Personally, it's the market cap. People see the marketcap of a token to buy or not, typically the lower the better (more upside potential). If it's held by the DAO, it's still technically existent so the marketcap will be higher, but if it is burned the fully diluted value (FDV) will be lower, thus becoming a more attractive token to buy.
In the long term it's good for the DAO to have tokens, but I don't believe the more the better. The DAO owning a portion of tokens is always at risk of bad governance leading to bad spending, or even a hack or something like that. Burned tokens are gone forever, no questions about it, so it's a bit safer in my opinion to have some tokens go to DAO and some burned, but never all to DAO or all burned.
So at least we agree that it was probably best to split the fees between null and dao.