This is what we do. For frontend positions, we give a laptop and a jsfiddle page to code up the answer to a technical problem that should take about 45 minutes. Then we leave the room and come back in 45 minutes and discuss how they did.
I know I tend to freeze up when writing whiteboard code in front of a interviewer, so I do not want to subject somebody to that when I am interviewing. When you are looking over someone's shoulder, you are not getting a true picture of their ability.
The other thing is that writing code on a whiteboard has nothing to do with coding in the real world. You write some code, test by running in a browser, command line or IDE, refactor, test again etc. Expecting someone to write perfect code without running said code is ridiculous.
Whiteboards should be used for sketching out ideas, i.e. architecting different ways to solve a high level problem, like how should I organize these objects in the application? What should the interface look like?
For large companies there are legal problems there. If you can't give every single candidate the same exact experience / ability to succeed in an interview, you are open to lawsuits. If candidate A got a laptop, but candidate B didn't have the option, B can sue. So if you can't guarantee you will have a laptop at 100% of interviews for a given position, you can't offer it at all.
That said I feel like interviewing is important enough that a large company with (presumably) large resources ought to be able to make sure a laptop is always there. But it isn't as simple as "just bring a laptop", you'd have to have an enforced process.
That's what I do. If the candidate is in the room I hand them my keyboard. If they are on a google hangout I send them a link to collabedit.com. People seem to do a lot better with a keyboard and syntax highlighting.
