If your using a mechanical typewriter, your likely not in a public (and/or pre-deterined place). This forces an attacker into a cumbersome, intrusive, and maybe-evidence-left-behind espionage scenario (B&E). That in and of itself is quite useful.

Wouldn't airgapped computers have almost the same effect? EM sidechannels are probably comparable to acoustic for espionage difficulty.

Or just shine a laser or RF beam at the window and read the vibrations.

you'd have to have either a permanent installation or exquisite timing. again, the greatly narrows down attack surface. you might be able to surveil small lots of targets this way, but it would be harder than you think. Again, if your a stuxtnet-level target, you're not going to be safe...but that's not really most people...is it?

Your normal-ish executive will have the ability to work in places without LOS from non-credentialed personell (eg, high rise apartment, gated community, interior conference rooms, etc). The NSA or MI6 could of course rent the flat across the street, or have a sub-contractor con his way into employment, but this type of stuff is outside the remit of your basic opportunistic threat. And it leaves evidence and paper trails unless you are the mossad or cia whomever and can fake identities like passports, bank accounts, etc.

Less effective if there's more than one typewriter in the room or if the curtains are drawn.