I still have that trap from my old Varta battery and I have just had to replace the Vartas replacement as it failed over the winter in the garage, with just two years on it. I got a AGM battery and I fitted the trap to it. Sure it should not leak but the gases will still be there and maybe some moisture as it breathes due to heat and the electro chemical process etc. I'll make sure there is a plug is in the other end of the battery as well.
That Varta still holds a charge very well, they are excellent batteries, it was made in 2004. It makes me wonder why I replaced it in the first place, as it never showed any sign of weakness.