basically the main unit always takes priority, when the tablet drains the dock charges it up at the same time, hence why technically its better to charge both tablet and dock seperately.
Just to clarify...
When the tablet is docked, the dock battery is used to power the tablet and the tablet battery idles unused. If the dock is not charged, the dock battery will drain to a few percent and then the TF tablet battery will kick in to keep it running until it drains. But if you connect the charger to the dock before the dock battery is depleted, then it will charge it back up and still not use the tablet battery.
If you always keep it docked and never drain the dock battery, then the tablet battery will never really get used at all. However, since Li-Ion batteries have a mild self-discharge rate due to the microchip in the battery and electron loss, the tablet battery will slowly drop from 100% over about 10 days or so down to 95%, at which point the charging circuit will top up the tablet battery to 100% again using the dock battery. The dock battery will then drop a bit for this charging process. You then charge the dock battery as usual. The tablet battery will go through this mild self-discharge cycle every so often and top itself back up so the tablet always has a "full" charge (a full charge is considered to be 96-100% capacity and is why the battery won't charge continually. The charging circuit waits until capacity drops to 95% before charging up again).
So personally, I like to remove the tablet from time to time and use its battery, reducing the regular drain on the dock battery. If not, then the dock battery will run its life cycle of X number of charges and begin to lose capacity, while the tablet battery is still good. There are many times I want the tablet by itself anyway so it's good to remove it from the dock once in a while let the dock battery have a "rest". This will extend the life of the dock battery allowing both batteries to extend the life of the TF in the long run.
Charging separately is not needed, but if you use them separately from time to time, and charge them separately, then you will extend the overall life of the TF unit. It's like having two batteries for the tablet - if you only use one all the time, then it will lose capacity long before the other reducing the usefulness of that battery.
If all you ever do is leave the TF docked as you like using the keyboard all the time, then a netbook would be a better choice, but you wouldn't have Android on it.