Its a bit hard to explain, but let me try.
So, you can place the slopes in the corner right? so that there is a 45° corner instead of 90°.
BUT the sides thar are on the wall are still normal squares-blocks.
so, if you would have a stair there instead of a normal block, the slope would stand out, since the stair is...well...stair-shape, it would not form a smooth connection.

So the suggestion would be in short, to implement a way to "hammer" a slope to make it form a stair or slab when its placed on a wall, just like you can hammer a normal block into a slab.
So its not about the 45° side to form a stair when placed on the ground (where it acts like a stair), just when you use it on the walls to make slab/stair-cornerpiece-slopes

Made a small sketch of what i mean:
http://i1331.photobucket.com/albums/w585/LazerusKI/Minecraft/Slope-Stair_zpsridahy54.png
From Left to right:
Normal Cube, Wall-Slope, Stairs, Slope-Stair
Lower Row:
Stair + Slope-Stair next to eachother

Not a terrible idea. Would be a little work rendering and lighting the faces though.