OSX just got SDHC support (albeit very slow read and writes till someone adds that to the driver too), so you might be able to keep plugging along with your 16 and 16 approach if you're not trying to run apps of the SDHC or play movie files off of it..
As for replacing the SSD
. Especially in OSX there are some good reasons: Sleep.. You can close the lid and put the machine to sleep, no need to shut the computer down when you're done and then wait for it to boot back up before you can start again.. The original SSD
that comes with these machines just will not wake from sleep with OSX. It's obviously a driver issue, but one that nobody has been able to solve, so the hardware solution is what many have gone with. And there's a plus to that anyways.. The second good reason to replace the SSD
applies to all the OS's.. Speed, the replacement drives, RunCore to be specific, are significantly faster than the stock drives. This makes the machine sooo much more usable.
Toss in the fact that you can go as large as 64gb so you can really install all the apps you want without concern about space, and it's really a win win deal.