We're talking about a default feature which is disabled 'for average users'. Average users only have a few tabs open in their browser. Not 100. Average users also don't have 3 GB RAM, nor do they use view source on a mobile phone, and neither do average users actually use the feature. So the default setting makes sense even though a user who is using the feature may end up using several MB due to a reload (not cool on a plan).

If you are saving 57 kB per tab open, that'd be ~5,7 MB with 100 tabs open. But if you have 100 tabs open on a mobile phone (!!), you have a bigger problem, and all those tabs are causing swapping already anyway. In that sense, enabling the feature by default makes sense. And don't forget that some people don't have flat rate internet.