I've never studied the AutoHotkey source directly, so I may be a little off the mark, but this is what I understand the situation to be:
It'd be more convenient to use, but the current way it works is very simple to implement and maintain. Essentially, any line that begins with an operator character (such as comma, dot, plus, minus, etc) is appended to the line before it before
further processing can proceed. This allows an expression, or the parameters of a command, to be spread across multiple lines. However, it isn't complex enough to recognize the difference between a plain text parameter and an object in an expression.
If quotes and braces were to be added to the list of symbols which cause implicit line concatenation, it would effectively break the behavior of closing braces, and for opening braces where the previous line does not support OTB. To implement the change you're suggesting would require a deep level change in how AutoHotkey treats individual lines and parses expressions.
Because of these reasons, it's much more effective to give AHK the hint that you want to combine those lines. This can be done, as previously demonstrated, by using a continuation section with the
is not technically required). Though I suppose if there ever was a time to make this unnecessary it would be now during the active development of v2