The case of USB connection upgrade comes with several benefits, perhaps the only issue if it can be called that way is when a 3.0 cable doesn't fit in previous versions. If you haven't noticed yet, a USB 3.0 female socket admits previous cables of USB 2.0 and 1.0 although not in all versions. In the two previous options, there was a mini USB and since the 2.0 a micro version. This one seems the preferred choice for smartphones and tablets today but it's true that the stability of the connection could be improved.
With the USB 3.0 standard, there's no more mini but an improved micro with an added section that helps provide better stability, robustness and more bandwidth. Due to the number of connections it seems that newer versions wouldn't need more sections thus the design of the connector should stay the same making our life easier after the interesting updates.
Because besides of a more robust connector, speed and current are other two main causes of the upgrade. With USB 3.0 the supported current goes from 500 mA to 900 mA per connector. If a normal charger usually goes at 1A (1000mA) or more, using for instance two USB 3.0 connectors the total current goes for a very interesting 1.8A in case of specific needs.
Better power efficiency? Yes, the devices can configure low consumption and power-off states really saving important amounts of energy.
The data transfer speed per second assumes some conditions that most of the time aren't possible or don't literally exist but the user is going to note a very fast connection up to 10x times faster —some state more— according to the increased demands in the last years. For giving you an idea of the speed, benchmarks for specific vendors should be accurate enough.
An example of a right upgrade in engineering terms and design!