You claim a more fair comparison is bitUSD versus Tether, but that is an unfair comparison. You can't compare Tethers to bitUSD... they are two completely different things.
One is fully decentralized, and pegged by free market dynamics on a decentralized exchange.
The other is fully centralized, and pegged by a promise from a company to maintain the peg on centralized exchanges.
There are some big differences there.
Even comparing Tether adoption versus bitusd adoption is flawed. Tether started with a large amount of private investment capital, and was able to instantly use that to create a large amount of liquidity. I speculate that it also used that to gain trading pairs on some exchanges, because it almost instantly became a "base" pair on some of the biggest exchanges trading against multiple cryptocurrencies on each one.
bitUSD on the other hand has to slowly and organically grow over time due to its free market based peg. As usership grows, market depth and liquidity will increase, which will also reduce the spread and volatility. But it will take a while to reach "critical mass" where the design works optimally.
The way to evaluate bitUSD is to look at its market cap compared to the total market cap of crypto, and to evaluate it over time. If the total crypto market cap doubled, there should be twice the need for bitUSD. If bitUSD as a percentage of total crypto decreases over time, then the market is proving that bitUSD is flawed.
By your own stated metric, bitUSD is actually doing pretty well.
Oct 2014:bitUSD:
Market Cap: $392,539
Bitcoin:
Market Cap: $5,241,480,575
Today:bitUSD:
Market Cap: $4,252,534
+1083%Bitcoin:
Market Cap: $41,066,547,811
+783%But your metric is flawed because it doesn't account for market dynamics. Right now there is higher demand for bitUSD and Tethers due to the bearish nature of the cryptocurrency market, as evidenced by ballooning market caps of both in recent time. When the cryptocurrency market is bullish, their market caps will shrink. This metric will be different if taken at a bearish or bullish time in the market.
Not to mention the combined network effects of all cryptocurrencies (and even Bitcoin alone) is exponentially larger than the network effect of bitUSD, therefore they will grow at a faster rate. Bitcoin's (and other cryptocurrencies) usage, value, and market cap growth is effectively 98% based on its network effect.
You keep citing that bitUSD is highly volatile, but fail to see that it is way more than stable enough for one of its main uses (which is the main reason why people use Tether too) is to hedge from cryptocurrency volatility or a cryptocurrency bear market. Comparison versus crypto was meant to show that it is quite stable over the long run... which again is backed up by historical data. It always comes back towards $1 even though there might be small fluctuations in its value away from $1, and thus it is a good asset to hedge against a bear market. It serves this purpose well.