Of course there's a complex interaction between letter and punctuation design there - if we didn't use capital letters, our full stop marks might be bigger instead. And perhaps that would be more elegant than having two versions of every letter. Now, that's of course a "what if ..." and not a practical argument that should affect your blog post today. (Nor is it an exhaustive argument, emphasized sentence beginnings aren't the same as emphasized sentence ends.)
But it's super-interesting that you bring up periods, because they're a great example of rules vs. the reality of written communication. The rules require ending sentences in periods, but a vast number of people start dropping them when the medium has an implicit full stop, like chats (IRC, SMS, ...) do (line/message end = full stop). Instead the period becomes repurposed as a sort of "aggression mark": http://www.newrepublic.com/article/115726/period-our-simples...
Technically the (tiny, one-pixel) period is there, but personally I cue off capital letters rather than punctuation when dividing sentences in my head. They are also helpful when skimming. They provide easy-to-spot anchors at which to resume reading.
Seriously, just look at what I've written. Blur your eyes and slide over the text. You can see the capitol letters from a mile away, can't you? While the periods fade into the page.