I can think of a couple of things, the most prominent is the power source. Turn indicators are usually electrically powered, whereas attitude indicators are vacuum powered, and the idea is to provide redundancy should either power source fail. This is not always the case though, and if your second attitude indicator is powered electrically, that's obviously a non-issue.
The second thing is the absence of a ROT indication, forcing you to calculate the correct bank angle for a standard rate turn should you ever really need one. The only situation I can think of where it is imperative that you can do a standard rate/half rate turn is a no-gyro approach in which case you've probably also lost your vacuum source, and you don't want to start playing mind games, but it's not as crucial as the power source redundancy.
As for whether it's acceptable, StallSpin seems to have that covered. The pictured second attitude indicator is fitted with a slip indicator, so that should cover that aspect.