Except voice controls don't always work.
It might not recognize your voice correctly (in my case, Siri only understands about 20% of my questions). Apple's voice recognition requires Internet access for many of its functions (limiting functionality for people with no or small data-plans, or using Wi-Fi devices where there is no open accesspoint). Some songs have difficult to pronounce names. Sometimes it may not be appropriate to talk (for instance, in a library). Or perhaps I just don't know the name (but I would recognize it if I saw it in a list).
Worst is that - even if the voice control does work - Siri is awful about actually FINDING the song. Too often I've asked iOS to find a song - giving it the exact name - and it tells me there's no such song on my device (although it helpfully suggests that I can buy it on iTunes) which means I have to manually find it in my playlist. Older versions of the app had no such issue.
Adding voice-recognition is a nice feature, but it's no replacement for a well-designed graphic interface, all the more so when the voice-recognition has such serious limitations.