Whilst I do find it irritating, I don't mind about the scope default so much, but I've certainly run into it (forgetting to specify) a lot.
Mostly because JS is not the only language that I'll be working with at a time, and if I'm dancing between three or four and they have similar syntax, then mistakes creep in.
Also, I like knowing which variables won't be reassigned (const), and which will (let).