Ahh I see, so this method is not really ideal? Although its using a little bit of the SD card - not much?
SUroot's the guy to explain the philosophy (it's his ROM), but his thinking is basically this: If you use an ext partition on your card to extend app storage, the size is fixed. So if you are only using part of it, you are wasting the rest of the allocated space, and conversely if you manage to fill it you can't extend without the hassle of repartitioning. The "move to sd" method however is dynamic, i.e. it only uses up the space that it needs, with no fixed limit. Hence it makes more efficient use of the card.
The real drawback with move to SD is that widgets don't work when moved that way. Thus apps with widgets that you intend to use should be left in internal storage.
I see, so this way you are 'opening up' the internal card even more due to a decrease in partition for the system files? Do you know how much gain there is (i.e what can the other folder size be increased to)?
Yes. The most extreme custom hboot, 60MB /system, 5 MB /cache, will leave 373 MB for /data (i.e. user apps and app data, what the phone calls your "internal storage"). Many of us actually use the 65 MB /system hboot, to allow a little more space e.g. for theming, which then gives you 368 MB. This is a gain of 220-225 MB over the standard partitions.
So ideally it is always best to have apps running internally but for some users they need more then what the internal offers? Therefore the SD comes into play but even then 100% of the app will never run on SD alone?
Are things like contacts and messages never stored on SD?
Contacts and messages are app data, and so those are always internal. You can back them up to SD (or the cloud), but the phone apps will always use the copy that's in internal storage.
Neither the "move to sd" nor "a2sd+" method moves the whole of the app to sd. I believe that other techniques, such as Data2SD, do, but there are more major performance drawbacks with those. I've never tried them, but many who have have reverted to the simpler methods.