IN App updates (Host files etc):
Basically because you are in an arms race agains the advertising / app developers.
The developers want you to see the advertising, as this is one of their sources of income. They are also aware of Ad-Block, AdAway, AdFree etc etc and are constantly trying new advertisers, new proxies and new methods of embedding the adverts in your application, website etc.
In order to keep the adverts blocked, the application needs to update the host files and lists of blocked sites, proxies etc to keep functioning correctly.
There should be a 'Check for Updates Daily' option, there definitely is in AdFree Android (well recommended) which you could use as an alternative to AdBlock Plus. By default most of these programs seem to check for updates when the device is rebooted from scratch, and have a daily / weekly or such checkin also.
You could go to
Settings > Applications > AdBlock and disable notifications also.
You could, I gues, just dismiss the notifcation when it arrives, and update whenever suits you. Leaving it go does mean adverts will start to creep back in over time, however.
Updates to the App itself:
If the actual application (i.e. the apk) itself has very regular updates then either the application needs to update to keep ahead of the advertising, which seems to be to be a bit of a flaw, or it is (as you said) in beta stages and needs constant tweaking. Alternatively with so many new devices coming online, and KitKat released it may need all this fixing to ensure compatability.
The simplest solution for either scenario, to my mind, is to get AdFree Android or similar and not worry about this at all!