Deus ex is, in my opinion, not always bad.
In fact, I see many climaxes being a 'Deus Ex' of one sort or another. In my mind, it's a continuum--a scale of how justified your climax was. A well-foreshadowed or well-developed sudden salvation when all seems lost is actually a very fulfilling plot device.
Tolkien is very much into Deus Ex endings, but I think he does them well. The movies take away from this, because of the aforementioned condensing, but there's still usually some foreshadowing.
Lets look at several of the movie versions, and put them on the scale. The best, I think, is Gandalf's appearance at the end of the Two Towers. This is a well-foreshadowed appearances; he even told them he would appear on the dawn of the third day. It works so well because we're anticipating Gandalf's return, we're reminded that we're anticipating it right before he comes, yet we are still uncertain how he will save the day. When he appears with an army we have forgotten, yet one that was very justified in appearing, there is a moment of glorious realization, and the day is saved. This is an excellent climax, though it is a 'deus ex,' since the heroes were saved by an outside force.
A lesser use of Deus Ex comes when Aragorn leads the ghosts out to smack down some orcs. This appearance is foreshadowed, true, but it is too convenient. The reason Sprig disliked it is because the story hadn't 'earned' such an easy end to the battle. We are left thinking 'well, if Aragorn can do that, then what was the point of all those other battles everyone fought?' He led too powerful a force into battle, brought it literally out of nowhere, and invalidated everything everyone else had done.