If you tried to keep those cities, you'd likely soon find them under siege, rebellious and tying up your resources by necessitating large garrisons. All the while the seleukids use their free scripted money to produce massive armies in the provinces that surround your newly-aquired territories. It's a very nasty form of over-extension. Instead, you can make it the ptolemies' problem and direct your armies to attack other seleukid cities while their troops are busy trying to kick the ptolemaic occupiers out.