IMO, LTC is already enough even they want to add more currency more preferer using TRX instance than DOGE maybe BSC also is okay for me since now everyone on cryptocurrency more often using BSC for trading on AMM exchange as well.
Litecoin is enough and Dogecoin can be considered if the team are free with workload and more important priorities for their gambling site.
Litecoin has a very strong and healthy network, only behind Bitcoin and Ethereum networks in total hashrates. It would be helpful for LTC network to be safe from 51% attacks.
Cost of 51% attacks from NiceHash estimation and that website has stats for network hashrates.
I have no problem with DOGE, but I think it won't change anything if the dev lists DOGE. Deposit and withdrawal via LTC is near free, so adding cheaper alternative is just too much work (with little impact).
Dogecoin is only lovely in emotion. Transaction fees with Litecoin or Dogecoin can be considered the same but gamblers can have more love for Dogecoin as they can use thousands of coins to bet. Just a feeling to bet with big amount in Dogecoin.
Anyway I agree with you that technically and financially, it does not make much sense.