AFAIK the main reason Dolphin needs to be GPLv3 is so that the Android version can continue to grow. If Dolphin stayed strictly GPLv2, then Android development would have to stop.
As for what the GPLv3 brings to the table that the GPLv2 doesn't have, AFAIK the main reason for the GPLv3 was to prevent "TiVoization" of software, which is so named because TiVo managed to use a loophole to violate the spirit of the GPLv2 without breaking the letter of the licence.
In short, the GPL says that if you use GPL software in your product, you can modify the software, but you must release the source code for any changes you make. TiVo uses Linux in its DVR firmware, so TiVo was required to release the source code for its DVR firmware (which it did); however, doing this potentially posed a problem for TiVo: if the source code for the TiVo firmware was available, then people could modify it to create custom TiVo firmware (e.g. to disable the requirement for having an active TiVo subscription) and then run that firmware on their TiVos. In order to prevent this, TiVo used special hardware restrictions in every DVR so that if a user tried to run unsanctioned firware, the TiVo would reject it. In this way, TiVo was able to technically respect the GPLv2 by releasing the source code for its firmware, but the company managed to prevent people from actually doing anything with that source code by making sure that no TiVo unit would let you run modified firmware that you built from the source code yourself.
The GPLv3 was devised to explicitly say that releasing the source code to your software isn't enough; you also can't employ hardware restrictions to prevent people from using your GPLv3 software.
As for what the GPLv3 brings to the table that the GPLv2 doesn't have, AFAIK the main reason for the GPLv3 was to prevent "TiVoization" of software, which is so named because TiVo managed to use a loophole to violate the spirit of the GPLv2 without breaking the letter of the licence.
In short, the GPL says that if you use GPL software in your product, you can modify the software, but you must release the source code for any changes you make. TiVo uses Linux in its DVR firmware, so TiVo was required to release the source code for its DVR firmware (which it did); however, doing this potentially posed a problem for TiVo: if the source code for the TiVo firmware was available, then people could modify it to create custom TiVo firmware (e.g. to disable the requirement for having an active TiVo subscription) and then run that firmware on their TiVos. In order to prevent this, TiVo used special hardware restrictions in every DVR so that if a user tried to run unsanctioned firware, the TiVo would reject it. In this way, TiVo was able to technically respect the GPLv2 by releasing the source code for its firmware, but the company managed to prevent people from actually doing anything with that source code by making sure that no TiVo unit would let you run modified firmware that you built from the source code yourself.
The GPLv3 was devised to explicitly say that releasing the source code to your software isn't enough; you also can't employ hardware restrictions to prevent people from using your GPLv3 software.