Undumgi have extreme synergy with spymastering externally. Getting establishing them means that we get to establish security over the foreign quarter of k8p (which we need to do as a spymaster), we get immediate access about all the information passing through what is to be a major trade route and have a shoo-in in establishing information network in the nearby Ulrican settlement (again, a necessity).
Undumgi can be leveraged in an increased presence of East Empire Trading Company, as there is world of difference between one of your founders being a dwarf friend living in the hold and one of your founders being in control (for some degree) of the foreign quarter of said hold. This can not only increase EETC revenue and profits but also give us easier time in making EETC a part of our information network, which both helps our general spymastering and lets us keep an eye on Stirland just in case.
Undumgi are likely to be much less sucessful without Mathilde. While they are steadfast and skilled in arms, they are not most organised or law-abiding bunch and are unlikely to have good institutional experience in building such organisations. Anything they come up on their own is likely to be corruption-prone and extortion happy, undermining the very idea of the organisation. Which is a pity, because they can be so much more.
Undumgi do not lock us into being a full-time mayor (unless we want to), which BoneyM graciously clarified.
Undumgi are time-critical. They are being organised right now one way or another, with us or without.