This is a particularly relevant question to consider, especially in today's day and age when those who decide on what grammar is acceptable and what precisely defines standard English usage. The problem is of course that what we can identify as being the standard of English at any one point in time is going to change, as languages do not stay still but continually develop and alter due to society, culture and globalisation. What is fascinating about this debate in our age is the way that globalisation in particular has impacted what we refer to as "standard English usage." How can we hope to pinpoint specific guiding principles when we live in an age where English is the most spoken language in the world, but where there is massive diversity in relation to how English is spoken depending on where you are and where you come from?
To give a small example, what is generally thought of as being "proper" English is sometimes called Queen's English, which is spoken by middle and upper class Britons. However, even within Britain, the rise of different sub-cultures and also the increase in certain groups of ethnic minorities threaten to erode what could formerly have been classed as grammatical foundations. I imagine the same is true in the United States due to the rise in the Latino population and the interaction between Spanish and American English. Therefore it seems far more pragmatic to focus on the kind of English that is actually spoken rather than trying to hold back the way that the language we speak changes. A good example would be the way in which "whom" is very rarely used nowadays, as most people say "who" instead. Should we lament the passing of this word? It is a difficult one to decide.