There are two things to consider concerning auto-power-up via plug insertion vs. discrete on/off switch, with the former usually being preferred over the latter.
With auto-power-up on plug insertion, you don't have to remember anything. The guitar is automatically ready to go. For performers especially, this is a Good Thing. Nothing worse than hitting that first power chord and having nothing happen in front of a big crowd. The downside to the method is if you're primarily a home player, you might often leave the guitar plugged in when you place the instrument on a stand or hang it on the wall. This leads to a dead battery in fairly short order. Perfomers don't usually have that problem because when they're done playing, the instrument generally goes into a case for transport which forces it to be unplugged.
The discrete switch is more intuitive and may be easier to implement in some situations. Plus, it's easy to see its on/off state. But, with this solution home players and performers both are more prone to forgetting to power the thing up or leaving it powered up when done playing, with the resultant battery drain. Plus, there's the risk of inadvertent power down during performance from fat-fingering the controls. Finally, it's yet another hole in the guitar body and more clutter on the face.