Don't get the main mod mixed up with those civs provided for scenario makers.
As a for instance, say I wish to duplicate the Norman Conquest of England... I really can't use the English as one of my civs. That's a post conquest civilization. The Normans would be more appropriate. But Normans vs post-Conquest English is essentially Normans vs Normans, so I need an Anglo-Saxon civ to represent Harold Godwinson and co. So in such a case, both an Anglo-saxon and Norman Civilization is needed, and an English civ just doesn't fit the bill (and actually, would be more appropriate as the Normans than as the natives, oddly enough).
Similarly, if I want a 500 AD scenario with the Romano-Britains against Anglo-Saxons, than I definetely want to see some sort of West Roman rump civilization included.
Normans are included as a seperate civ from France as they weren't French, but a nobility transplanted from Danish raiders, Rollo the Viking. They went on to many conquests divorced from the French crown such as Sicily and Naples, and England. They adopted French culture and language, however, the Norman knight, with his distinctive head gear, was different from the French, and the far reaching overseas ventures of the civ are more reminiscent of their Viking forebearers than the French.
Many other duplications have been alluded to, as well. The Anglo-saxons came from Saxony, Friesia and the Jutland peninsula. The saxons, named Wessex, Essex and Sussex (plus others) were from Frisia and Saxony (the Frisians, themselves, were essentially Saxons migrating there during the Germanic migrations). The Jutes, from Jutland, went to Kent and the Isle of Wyte (sic.). The Angles did east Anglia, Mercia and Northumbria, to name a few. The were from the Southern Jutland peninsula and Pomerania. These were all Teutonic peoples with similar language and culture and origins. Any distinctiveness they had pre-invasion was lost when the petty kingdoms in England were united, so treating them as the Anglo-Saxon civilization makes sense, even in the context of German Saxony, where the Saxons were absorbed by the Frankish Empire (to come out of that process as the Holy Roman Empire led by Otto I of Saxony) to later become an essentially German Civ. For the time period, 500-800 AD representing Anglo-Saxons and Saxony with the same UU's and similar leaders makes sense.
The Visigothic Kingdom in Spain lasted quite some time into this scenario. That being said, it might possibly be included amongst the catch all of Germanic peoples.
I kind of agree that there should be a civ seperate from the Berbers to represent muslim Spain.
Anyhow, the idea of the mod seems to be to represent 16-18 civs in the main scenario that best cover the characteristics of the main civ actors during that period. Granted, almost no particular civ lasted throughout the whole of that period, unchanged (except for the Byzantines). So France will have to, in the mod scenario, stand in for the earlier Franks, England for both pre and post conquest island inhabitants, Italian states as an amalgam of later northern Italian city states (including Venice and the Papal States in the mod scenario), Spain for its sub-kingdoms, Germany for the East Frankish/Saxon Holy Roman Empire, and so forth.
The main mod is one thing, but civs included as extras are essential to scenario makers, and need to cover a lot more specific civs then the general mod. That's why there are Anglo-Saxons and English, and why there seems to be some duplication.
The mod runs from 500-1500 AD. I personally think that it makes more sense to call civs by the names of nations that they will later morph into. By 1500, there are no Lombards, nor Franks, nor Anglo-Saxons... so I'd rather play them as the Italian States, France and English... how they end up. Historical accuracy suffers somewhat... as I mentioned, the Anglo-Saxons are definetely not the English, in civ terms, they lost, but historical accuracy is always going to suffer either way you go. So if you're going to be inaccurate, you should at least be consistantly so... all late medieval or all migration period.
I reiterate, though, don't get too wrapped up in the 30 civs or so that will be chosen. If you have arguements about inclusions for historical reasons, then I'd be most concerned about the 16-18 of the main mod. The others are meant to cover more specific scenarios. Including them doesn't affect or detract from the main mod in any way. They don't necessarily have to balance with the main mod scenarios civs, and can be inconsistant and duplicate them somewhat. Many of them will actually not be seperate from the main civs, but will be in the form, rather, of leader options. This is specifically for those civs that Head Serf mentions with (...) around the sub-civs. Vikings will include, I hope, leader heads for Canute, Olaf, and Hakkon, for instance, that can be used for seperate Denmark, Norway and others. But an actual civ is not needed for each of these seperately, just leader head options. Similarly for France, Franks... a Charlemagne leader head will serve adequately for both and if having Charlemagne as a French leader bothers you, then don't choose him when you play the mod, and if you make your own scenario, don't put him as the French leader.
I've gone on longer than I intended, but I want people to be aware that there are two communities of individuals that this mod addresses... those who will play it as is, and those who wish to make scenarios and play them. Both can be satisfied by inclusion of additional, non-basic mod civs. As I said, any large arguements about the civs included should address the basic mod, not those included for scenario makers.
Note: on rereading this post, I find I say "you" a lot. By you, I don't mean any specific poster, but the general "you" as referencing people playing the mod.