I totally understand what eyald needs; I'm struggling for this feature as well.
I've compared CSP, PS, SAI2 for the similiar function (sorry for my odd English, and I had changed some hotkeys myself):
As for now I prefer the straight line mode of PaintToolSAI Ver. 2, it's the fastest software for drawing straight lines... (I also have CSP and PS) But aside from that, Paintstorm Studio is more powerful in almost every way.

I'm recently migrating to Paintstorm, and I'm relying on a plugin called Lazy Nezumi to achieve similar feature. But as a plugin it still has some flaws.
So...why is it so important to draw straight lines "without" a ruler?
Simply by pressing or holding ONE key, I can immediately draw straight lines supporting pen pressure (in any desired angle). No need to adjust the angle of a ruler. No need to call out the rulers states to find the right one to use. This is incredibly useful when "sketching", especially when making comic pages (or making designs). Because I need to constantly adjust my ideas and compositions of the perspectives, mechanical objects, furniture and weapons etc.... tons of things involving straight lines in lots of different angle, in every pages.
I've also posted some sketch pages (made in SAI2 and PS) as an example:
https://imgur.com/a/Zs7vK
Let's imaging what if all those straight lines doesn't support pen pressure? It'll be ugly. Or drawing all of them using rulers? It'll be time-consuming.
In this case, to make the steps as simple as possible, is actually much more important than to use rulers to draw "super clean and perspectively-correct straight lines". I appreciate the ruler tool right now, but it seems not too friendly for those users who are dealing with complex drawings. (And comparing Paintstorm to CSP, I actually like Painstorm a lot more. Photoshop? Forget it.

)
Suggestion:
- "Press" shift to switch on/off straight line mode.
- In straight line mode, every stroke will be a straight line in any desired angle.
- When not in straight line mode, "hold" shift key and draw to get 0, 45 & 90 degrees straight lines.
-
Always support pen pressure.
If this feature isn't deemed to be useful for enough people or it's impossible to code, it's fine, I still can rely on the plugin anyway. I just want to put my thoughts about this problem as clear as possible. Sorry for the long post and thank you again for making this wonderful program!