Are the spelling and grammar mistakes in phishing emails done on purpose? Is there some wisdom behind it? Or they are simply indicative of the fact that they've been written by someone who does not natively speak English?
This may well be for the same reason as many scammers rely on the tired old 'Nigerian Prince' strategy: by self-selecting for gullible targets, they can be more efficient.
In phishing, as in scams, sending the initial batch of emails is the easy part.  The hard part is coaxing information out of the target (which can require a concerted exchange of emails).  That can represent a significant investment of time.
As a result, it's really important to ensure that the people you correspond with may actually give you the information that you're after.  It can therefore be advantageous to send a badly-drafted email, on the basis that the people who respond to that are likely to be gullible enough to be phished.
(I would probably draw a distinction between these broad, drag-net approaches and targeted phishing, where you're much more likely to see carefully-crafted and legitimate-looking emails.)
