By definition, a dossier is just a series of documents (memos) compiled on a person's background and a history of that person's dealings with others.....
Steele was once (and may still be)a trusted ex-spy who had various contacts within the Russian political and economic sphere.....Like him or not (perspective) he was trusted enough by the FBI to ....at least.....be given an opportunity to examine his dossier.
Although little has been confirmed in what the dossier alleges, it will be up to Mueller's investigative findings to collaborate (or not) any of the dossier's conclusions among various other sources of possible wrong doings by the Trump entourage.
However, here are THREE points to be made about one's perspective in viewing the validity of the dossier..
One.....if one does NOT believe that the Russians meddled in our elective processes and possibly illegally hacked our systems, then the dossier has zero validity.
Two....if one does NOT believe that our FBI and other U.S. intelligence gathering agencies are professional enough to forgo political biases, then the dossier has zero validity because the FBI (especially) has pursued items alleged in the dossier.
Three.......if one does NOT believe that Trump has special links to Russian business deals and an affinity toward easing sanctions with the Russian government ...its ambitions and economic objectives......then the dossier has zero validity.
Much....one way or another....will be determined by the Mueller's investigation final report.
As a highly respected former member of the CIA’s Senior Intelligence Service, John Sipher (who examined the Steele dossier) stated:
Professional investigators can marry the [Steele] report with human and signals intelligence, they can look at call records, travel records, interview people mentioned in the report, solicit assistance from friendly foreign police and intelligence services, subpoena records and tie it to subsequent events that can shed light on the various details. We, on the other hand, will have to do our best to validate the information at hand. Looking at new information through the framework outlined in the Steele document is not a bad place to start.