As you mentioned, Vim already supports a mode where the window size is not a strict multiples of character cell sizes, when maximized. There are special cases in the code base to handle that already, and for the most part maximized windows look fine. If you do "set guioptions+=k", and then change font size, add/remove tabs etc, Vim is also going to keep the window size fixed, and display some empty space along the borders. This is just going to make regular windowed mode support that in MacVim as well.
I really think it will look more natural once it's rolled out. Let's say your monitor is 1080 pixels tall and your font size is 14 pixels, that means you will have 77 rows = 1078 pixels tall (I'm ignoring things like windows border here). You still need to fill those 2 pixels somehow. The current GVim (which MacVim is doing) is to let the desktop or whatever window below the GVim window show through, which is distracting. If you let Vim's window size be more flexible, you can fill those 2 pixels with GVim itself which just draws the background color (this part is implementation specific) in the bottom right and will look much more seamless. Also, in general, I think a native application should try to respect the window manager's will as much as possible.
Proportional font: Sure, I think that's a different discussion. The issue here is mostly whether we want GVim to be able to fill the empty space to the side (we are not changing the actual character cell size here).