Someone has probably already brought this up, but after Martin sacrificed himself shouldn't it be harder to conjure creatures from Oblivion? "On Conjuration" implies that the barrier between Mundus and Oblivion was weakened over time, due to the experiments of the Direnni mages, and after a while it become "impossibly difficult to shut". But if Martin's sacrifice makes the Dragonfires irrelevant, those smaller ruptures in the barrier used to conjure creatures would also be renewed, wouldn't they?

If I remember correctly, properly inviting a Daedra to the Mundus bypasses most of the wards. Actual conjuration is probably more of a brute-force approach since you're forcibly pulling a Daedra from Oblivion, but likely takes advantage of some of the same loopholes. The Dragon Fires are also just one defense, along with things like the Towers and possibly general faith in the Divines. Several Towers have been destroyed or rendered inert and this is likely the main reason the barriers still aren't perfect. While not nearly as often as in Morrowind, we do see Daedra manifest without mortal conjurers in Skyrim, like at Mehrunes Dagon's shrine or along the coast of Solstheim, though they are likely getting assistance from their Prince in both cases. The Princes also seem to be communicating, producing minor manifestations, and maintaining their artifacts just as easily as before, which to me is good evidence that the new statue helped, but isn't enough.

As Haskill points out, Martin's sacrifice prevents Daedra from opening stable portals for the purposes of sending invading armies through. There are still loopholes which clever Daedra can exploit, though.