happy to answer! I use it because it is what i know how to use. I tend not to use things I do not understand, just so I do not run into any unexpected behavior.
I am unfortunately not that capable with tiddlywiki, it takes me some time to learn to use newer features, and since I rarely have the time to sit and tinker with tiddlywiki outside of the occasional stylesheet, I unfortunately haven’t used many of the newer things like the if then else things with the angle brackets and percentile symbols, for example.. 
you are very much correct, with my personal favorite being kara, but I wanted to make something of my own, i find it to be the best way to learn, and even come up with my own new ideas.
also, I haven’t seen anyone’s solutions have the ability to add tasks like you can this way.
i.e. “Just finished work and came home, going to need to <<task "take out the trash">> after I get settled.”
Just wanting to be able to display a task inline with normal text, or to just type them one after another and show as a single collection of checkbox items, rather than each one being in their own <p>
i.e. this is the result of what I originally shared at the start of my post, when I use the internals plugin to view the raw html:
<p>
<label class="tc-checkbox ">
<input type="checkbox">
<span> <a class="tc-tiddlylink tc-tiddlylink-missing" href="#Clean%20litterbox">Clean litterbox</a></span>
</label>
</p>
<p>
<label class="tc-checkbox ">
<input type="checkbox">
<span> <a class="tc-tiddlylink tc-tiddlylink-missing" href="#Fold%20laundry">Fold laundry</a>
</span>
</label>
</p>
<p>
<label class="tc-checkbox ">
<input type="checkbox">
<span> <a class="tc-tiddlylink tc-tiddlylink-missing" href="#placeholder">placeholder</a></span>
</label>
</p>
(I indented the code above for easier reading, it just shows as one block of text otherwise.)
I was able to replicate that by typing out <<task 'placeholder'>><br/> but when I tried to add it within the procedure or even placing the procedure inside of another one specifically for adding the break row after each one, I couldn’t get it to work 
I imagine if I do manage to include it whenever I call it I wouldn’t be able to use it inline without it putting the text after it on a new line though, huh?
I might just stick to manually typing <br/> after each one.
Below does not work
\procedure task-item(task-item)
<$checkbox listField="completed-tasks"
checked=<<task-item>>
default=""> <<task-item>></$checkbox>
\end
\procedure task(task)
<$transclude $variable="task-item" task-item=<<task>>/><br/>
\end
\procedure task-item(task-item)
<$checkbox listField="completed-tasks"
checked=<<task-item>>
default=""> <<task-item>></$checkbox>
\end
\procedure task(task)
<$transclude $variable="task-item" task-item=<<task>>/><br/>
\end
<<task "Clean litterbox">>
<<task "Fold laundry">>
<<task "placeholder">>
Below does work though
\procedure task(task)
<$checkbox listField="completed-tasks"
checked=<<task>>
default=""> <<task>></$checkbox>
\end
<<task "Clean litterbox">><br/>
<<task "Fold laundry">><br/>
<<task "placeholder">><br/>
(Also, why does a procedure have you use <<this>> over $this$ for variables within it? What happens if you try to use <<currentTiddler>> instead of $currentTiddler$? sorry if that seems like a bit of a dumb question- I’m not used to doing it is all
)
edit: corrected some incorrect code, the new talk.tiddlywiki editor was messing things up when I was replying from my phone 