Well, before they can buy bc they need to take the shitcoin and trade it for btc then to bc incurring 2 trading fees. It makes more sense to mine with a multipool that mines shitcoins for btc, then you get a choice whether/when to buy bc. I think the idea was stupid from the beginning, not sure how to explain the enthusiasm shown by the current pump.
I've been playing with some of these types of multipools. I think they're good for die-hard followers of a certain coin that is not BTC who do want to spend the time trading their BTC manually. In theory, you get that done for approximately the same fee structure. So you do gain value by these pools.