It isn't the best proxy though, because it's not even trying to simulate the conditions under which real work is done (e.g. in real work, most developers will often look up details, and it's no problem at all - what matters is how they think about solving problems, that they know what they don't know, and that they fit into the team). This is a proxy for 'is this an experienced person with a strong memory?', not 'is this a qualified candidate who would perform well in the position?'. It's quite possible to fit the latter and not the former. If you're the size of e.g. Google so your problem is more to weed out bad candidates quickly rather than catch all good candidates, then sure, go ahead (false negatives are acceptable to completely avoid false positives). If you are a smaller company, then your pool of applicants is quite limited and it's much more important that you don't pass over any qualified candidates that happen to fall into your net.