Well how exactly can one fight a battle against Modernism and the Novus Ordo establishment other than by bringing souls to Tradition and to the Fulness of the Catholic Faith?
If the FSSP priests are validly ordained Catholic priests (since this is not the "Crisis" section, let us refrain from debating this point) then that is exactly what they do. While one may argue that Rome is just offering tradition a "side altar" in the abomination of desolation, it is undeniable that many souls would have never even heard of the Latin Mass were it not for the installation of a FSSP apostolate or a FSSP mass being said in a local church.
Because it is on regular terms with the Ordinary, the FSSP receives much more advertistement than the SSPX. For example, never have I heard a non-Catholic mention the SSPX chapel in my diocese - however, on one occasion I spoke with several that knew of the local FSSP parish (it also helps that this parish, due to it's excellent schola was featured on the front page of the Local section in the Sacramento Bee). Every year the parish leads a May Procession through town and offers a Solemn High Mass at the Cathedral (for the past two years anyhow - they are trying to start a tradition) - undoubtedly, this exposes countless souls to tradition. A great majority of the people that attend the procession don't stay for the mass, of those that do, many are confused and even irrerevent, but there is always a chance that seeds of conversion may be planted in 1 soul.
While many FSSP priests may not "denounce" the Novus Ordo from the pulpit, they can work in other ways to convert souls to tradition. The most obvious way is by offering an alternative to the banal New Sacraments. FSSP priests give solid guidance in the Confessional and in their sermons (which can be heard on Audio Sancto - these are typical of the sermons i have heard at the FSSP). The bookstore at my parish sells many Angelus Press books, expositions of the New Religion, and even the biography of Msgr. Lefebvre. The storekeeper refuses to carry Theology of the Body, etc - true, there may be post-Vatican II books sold (none heretical or sentimental) and true, they may have a bit of the "JP2 fever", but I doubt that there is another place in the entire diocese where you can buy the Syllabus of Errors or Pascendi, etc.
As to vocations, the FSSP seminary is flourishing to the extent that they need to expand their building. I've heard from a priest recently ordained that there were seminarians there that had only been to the latin mass once or not even at all, but were so convinced of the verity of tradition that they refused to seek priestly discernment elsewhere.
Whether or not this is significant...