More importantly, it says who they are NOT related to, with a certainty of 100%. That is the power of the tool. To ignore that, is just plain ignorant. So, if your mtDNA does not match the Queen Victoria maternal lineage, you are NOT a child of Alix of Hesse. End of discussion.
Well, actually... Rob...
While your words do reflect the popular view, your stated view above does, in fact, have a couple of flaws.
First..
With nuclear DNA there can be only two possibilities. Either there is a match or there is not a match. The same is not true of Mitochondrial DNA. In Mitochondrial DNA comparison, there is also a third possible outcome that can leave you with no answers at all.
If a single nucleotide is out of place in a nuclear DNA (STR) comparison, that would be considered a negative result. A nucleotide out of place in Mitochondrial DNA, however, may
not be considered to be negative. A nucleotide out of place in Mitochondrial DNA is, instead, considered to be inconclusive and worthy of more testing before any conclusions can be drawn.
A number of things can cause this to happen in mtDNA testing -- for reasons which have
no bearing on the identity of the person being tested. Foremost are the possibilities of recombination and heteroplasmy, which can easily skew the results, without having anything to do with the actual identity of the subject.
From: The Journal of Law and Policy, "Nuclear and Mitochondrial DNA in the Courtroom" by Julian Adams, Ph.D., Biology, Mar. 14, 2005:
"Mitochondrial DNA analysis is fundamentally different from nuclear DNA forensic analysis. In nuclear DNA analysis, a comparison of the two samples can lead to one of two conclusions: either there is a match, resulting in inclusion, or there is no match. In mitochondrial DNA analysis, the lack of conclusive evidence for a match is a third possible option."
"While population sample sizes will increase, resulting in a proportional decrease in the frequencies of mitochondrial DNA profiles, they will never approach those of nuclear DNA.... Recently there have been reports of a paternal contribution of mitochondrial DNA, so-called "paternal leakage", and the formation of new chimaeral (recombinant) genomes incorporating both maternal and paternal sequences. It is imperative that the nature and extent of these phenomena be confirmed and investigated. In the absence of further information, it is inevitable that these reports will stimulate new challenges to the admissibility of mitochondrial DNA evidence."As it has been said here numerous times before... Mitochondrial DNA is only an indicator. It is not proof.
The second and more serious problem with your statement is this:
It is entirely possible for someone who is not a Romanov to make a successful claim because it is entirely possible to have the same mitochondrial DNA profile as Alexandra and her great nephew Prince Philip... without actually being related to anyone in Victoria's matrilineal line.
The perfect example of this possibility is the Oxford DNA professor Dr. Brian Sykes, who has the same mitochondrial DNA profile as Nicholas II, but without the heteroplasmy at position 16169. Dr. Sykes has the same mitochondrial profile as the Tsar's nephew Tikhon Kulikovsy (who does not have the heteroplasmy), even though Dr. Sykes very clearly is not a Romanov.
In his recent book about mitochondrial DNA titled "The Seven Daughters of Eve", Professor Sykes has explained that his own Scottish-born mother could have claimed to be a Romanov and no one would ever be able to prove her wrong because her mtDNA is a match with the maternal line of which Nicholas is a member.. even though she is clearly not related.
All it would take is a common maternal ancestor at the top of two very different family lines... an ancestor who could have lived as many as a hundred generations or more in the past.
So, just imagine...
You continue to insist that mtDNA will prove if someone is not a Romanov... and then... along comes another "Anastasia" who actually does have a proven mitochondrial DNA match with Alexandra.. but is clearly not related to the last empress.
If you cannot prove that the next fake "Anastasia" to come along was not a Romanov because she does have a mitochondrial match with Alexandra... only by the mere happenstance of having a common maternal ancestor who had lived a hundred or more generations ago...
Then what?
How would you ever manage to wriggle your way out of that one?

jk