Let's take this document. 

<p>foo</p>
<script>
var a = document.querySelector('p');
var b = 0;
console.log(a);
</script>


Steps to reproduce
1. Load it in STP 179 for example with "data:text/html,<script…"
2. set a breakpoint on the line starting with var b
3. Choose Log Message in Edit Actions
4. Choose continue after the breakpoint
5. Enter ${a} for the log Message

Result:
When executing a reload, the log message from the breakpoint is 
[Log] [object HTMLParagraphElement]
while console.log(a) is <p>foo</p>


With ${console.log(a)} I get 
[Log] <p>foo</p>
[Log] undefined

With ${console.dir(a)} I get 
[Log] <p> (with an arrow showing all the properties, and the object already opened)
[Log] undefined


I'm not sure which one is the most useful, but 
[object HTMLParagraphElement]
seems to be the less useful. 


Also probably in the other cases the undefined is also not useful.