There is no evidence right now that the intelligence services can remotely activate webcams on any device of their choosing. They would likely need to first install malware on most devices to accomplish that.

http://www.washingtonpost.com/business/technology/fbis-searc... The FBI has been able to covertly activate a computer’s camera — without triggering the light that lets users know it is recording — for several years, and has used that technique mainly in terrorism cases or the most serious criminal investigations, said Marcus Thomas, former assistant director of the FBI’s Operational Technology Division in Quantico...

They can easily do that, that isn't really in doubt, though of course that's not why things have cameras, as you say, cameras are cheap and quite useful, no need for a grand conspiracy.

I'm sure they wouldn't bother to exploit a machine unless they decide to target you specifically, but if they do decide to, given the resources they have available and the imperfect security on almost every OS, network and hardware you might use, they can easily use your phone or computer to record by installing malware. It's really very straightforward, all they need is a zero day in any of the network connected applications you use, baseband of your phone etc.

William Binney (worked on Stellar Wind) and Edward Snowden have gone on record confirming these capabilities are used routinely - in particular I remember Binney confirming mobiles used as mics, even when turned off. For him this is a matter of fact capability, not at all as dangerous as mapping all our activities at once from metadata and content sweeps on a broad scale - I tend to agree.

http://www.nytimes.com/video/opinion/100000001733041/the-pro...