Royal genealogy has a bad rep which is unfair. Royal genealogy is pretentious, there is no doubt. But, isn't all of genealogy pretentious? We all talk of the revolutionary war soldier, the Mayflower passenger, the Ellis Island immigrant, all with pride and not a little braggadocio. We like our ancestors, even the bad ones.
The other bad rep for this type of genealogy is the thought that it is untrue. Or more to the point that it doesn't hold up under strict genealogical scrutiny. This discussion started when one blogger correctly noted that lineages back to Adam are bogus, but then threw out the baby with the bath water, and said royal lines are bogus too. Several other well-known bloggers have said similar things. I think this is a case of once burned twice shy. I have a page with all my bogus lines which far outnumber the lines I can prove. But I can prove those other lines--and so can you if you want to. I've actually done it twice from scratch. Rose (Stoughton) Otis and Anne (Skipwith) (Goforth) Oxley.
There are many primary records that can be used in medieval genealogy, far beyond secondary sources that give lineages. There are wills, fine rolls, patent rolls, close rolls, letters, land records, and contemporary accounts, all of which predate parish church records in England. Using estimated dates I would say that from 1520 to the present you have parish records and the usual sources, and the aforementioned set of sources take you from 1520 back to about 1200 (give or take). From 1200 back one is mostly at the mercy of land records where they survive and contemporary accounts. These accounts are called chronicles and they have to be interpreted not just by what they say, but who wrote them, and what axes they had to grind. However, this is the stuff of historical research.
Using a genealogical proof standard, can you prove that Julius Caesar existed? No. Everything we have is a secondary source that tells us that he existed. We have accepted these sources as sound. Likewise, in medieval genealogy, we use chronicles as well as primary sources to build a picture of the relationships of nobles and royalty. Sadly, that's all we can do at a certain point. You can push landed gentry and wealthy merchants back to 1400, and maybe to 1350 if they owned land. But you can only be certain of nobility and royalty at that time period and further back. [The average yeoman can only be traced to 1500 mostly--and that's being generous]. These are the only reliable records that are extant. So, royal lines can be proven and paradoxically if someone tells you they can put your Smith or Warren or Jones ancestor in the Domesday Book, that's the dubious claim. Trust me, your farmer ancestor did not come with the Conqueror.
Some royal lines lead back to ancient lineages, that is those that pre-date Charlemagne (742-814). There is great debate as to many of these lines. One famously disputed line takes a Spanish king back to the Prophet Mohammed. Sadly, we don't know which of his wives were mother to any of his children, so most sound genealogists do not accept the line. There are many other such lines that try to tap in to lines back to the Julio-Claudian emperors of Rome, Cleopatra, and the Davidic branch of the royal line of the Kings of Israel. That gets you back to the Bible and the genealogies contained therein, and if you believe those genealogies, back to Adam. Again, no genealogical scholar accepts such lines.
I pursue medieval genealogy simply because it is the one area where I can continue to research my own ancestors. I've hit all my brick walls and I have spent decades on them. They are not researchable. I can either give up researching my own family or I can push back through lines that are not brick walls. Most genealogists after a while hit such a plateau. They then research their spouse's ancestry; their in-laws ancestries; their friends, etc. anything to keep going. Medieval genealogy is one of those outlets. And because of how the records now exist, you have little choice but to confine yourself to royal and noble lines.