Somebody needs to. First, consider that every decision like that you make is multiplied by the number of clients you have. If your tiny little JS side makes your clients' CPU use one more watt and you have a million visitors, then boom, one megawatt goes down the drain. Maybe not something that should keep you up at night, but still IMO worth thinking about.

Moreover, your website isn't the center of the world and the most important thing for user. If every developer follows this line of thinking, then the difference between 5% and 10% of user's CPU time is the difference between them having 20 or 10 browser tabs open before the whole computer slows down to a crawl. Bloating your site means making your users' computers less useful.