Staking yield is paid in ether (ETH). So, in theory, a user could earn more ETH but, in real terms, that gain could all be lost quickly if the value of ETH were to take a dive.
However, though ETH's price increase has been slower than BTC's, it's been trending up over the last year.
Which means in real terms a staker is getting a double boon: both the increased value of their underlying ETH, plus more ETH.
The intrigue: Metamask has something of a captive audience of cryptocurrency users. Allowing them to join a staking pool with just one click must be attractive.
However, the company is not offering a tokenization of their stake, like other products such as Lido and Rocket Pool do. So Metamask users won't be able to get the double benefit of participating in re-staking.