The long line forms daily at the embassy for those seeking tourist visas to the US. And the list of denials is also lengthy.......often times the decision having been made long before you get to the window. To set yourself apart imho, and in reality, is a key element in increasing your odds of obtaining a visitor/tourist visa.
Tourist visas are granted for longer than 15 days. Most are for 10 years. This means the holder could ostensibly enter and reenter multiple times in one year.....but cannot legally stay after a continuous 180 days.....except under extraordinary circumstances...illness, etc.
As I indicated in my prior post, timing also plays a crucial role in approvals. How many times have people known someone they thought had a snowballs chance in hell of getting one......only to be approved. And still others who appeared to be a slam dunk are denied. I have seen both cases. It is, along with your documentation, very very subjective. Notwithstanding that, and the embassy will of course deny that they have a quota for tourist visas......but my experiences would suggest otherwise. Therefore knowing WHEN to submit is often times as important as all the other formal criteria.
So to finally answer your question, you should submit any and all documentation, including an LOI, that supports/bolsters your case...... and also recognize the unwritten quotas and the subjective nature of the process.
Respectfully,
Playacaribe2