Well, the aim is to write up and present a balanced essay which gives a contention. I think to have a more descriptivist tone means to widen your view of language so that it encompasses changes, rather than seeing it as dictated by rules (like a prescriptivist). So for the example 'We should all speak Standard English. To what extent do you agree?', think of the ways that Standard English benefits then the ways that non-Standard English benefits. Looking at it from both perspectives can help set a less prescriptive tone in your essay.
So potential arguments for could be:
- Mutual intelligibility
- Overt prestige
Potential arguments against could be (or, for non-Standard English)
- If we all speak the same variety, then it becomes harder for someone to express their individuality through idiosyncratic language use
- Cultural identity (e.g. ethnolects) - Non-Standard English is often a marker of one's belonging to this
I'll try writing up an introduction.
We should all speak Standard English. To what extent do you agree?
Language is a versatile tool which comes in a number of varieties, one of which is Standard English (SE), that aims to successfully serve a number of social purposes. Through its widely understood nature, the SE variety can help lower communication barriers for a wider number of individuals across different speech communities, hence acting as a world language used for international exchanges. Additionally, SE establishes overt prestige, thereby assisting exchanges in business and other official situations. Conversely, by having everyone speak this variety, there will be less opportunity for one to reflect their cultural heritage and express their linguistic creativity, both of which often contain features that deviate from SE. Whilst Standard English is pivotal in effectively allowing conversation in many situations, it should only be used to a certain extent due to greater preference for non-Standard English in other circumstances.
For your second question, yes you can and that's usually how comparisons between non-Standard and Standard varieties are done.
Hope that helped!