Creating a letter for one constituent is a hassle because you have to go into the Letters section of Bloomerang and change the filter every time you need to run one letter for one constituent. (These are letters outside of transactions.) If you could just generate a letter from a constituents timeline, the assumption is that they would be the only recipient and it would save you a lot of time.
This feature would be VERY helpful when I need to generate "Promise to Give" confirmation letters in conjunction with independent audits. These are randomly chosen pledges by a specific constituent.