Currently there's no problem using it for both since most of the products and service being paid for in bitcoins have their value priced in fiat. That means the amount paid is adjusted anyway.
If we intend for bitcoin to be the successor to fiat though, then it'll have to be less volatile as it is now.
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That's not very likely
Since it is a vicious circle of sorts. Using Bitcoin as a payment device or value transfer vehicle means real value, while increasing real value leads to less volatility. On the other hand, speculation boosts volatility and prevents mass adoption for Bitcoin's use in these fields. As speculative value comes before real value (it just happened so for whatever reason), it prevents adoption but without adoption there is no way real value is going to increase. Thus you get locked into this vicious circle
Sadly yes.