I absolutely agree with knowing what Poloniex's market cap is the same way we know what the Market Cap of say the NYSE is at any given time - for example we know that 1 trillion dollars evaporated from global equity markets just this week.
We should be able to determine market cap for all of the exchanges and this would give individuals a clearer picture of their 'broker'. Alas the market is unregulated and they are not compelled to display this for us.
I enjoyed your post, its a very important point. I tend to gravitate towards the exchanges where there is the most volume and therefore liquidity.
Conversely, the difference between Polo/Kraken and bittrex is that they carry fewer coins, likely risk management. So my real question which exchange is exposed to greater risk from the failure of a coin.
Thanks Jerry!