That is a good point if you do a lot of travelling. I generally don't so it isn't such a problem. As for battery life, I've managed to get my PSP lasting up to 10 hours on a full charge as I run games and emulation all from memory cards so it isn't draining the battery accessing UMD discs.
I think with the latest custom firmwares you can now use 8GB Memory sticks. At the moment I have a couple of 4GB Memory sticks which can fit a lot of software and roms on them and are more than enough for my current needs.