It depends on the size of your world. In my world, it takes around 7 minutes to walk from my main base to Mount Vulcan, and whenever I realise that I need some resource that I left behind in my main base, I can just pop into the Nether, collect it and be back at Mount Vulcan in no time. Even the best railway system wouldn't be that fast. It also allows me to connect up outposts that I haven't been able to connect in the main world yet, such as my snow biome outpost.
My world's longest journey by rail is about 9mins (from far to far end), it's pre-biomes so I needed walk for ages to find snowy and sandy areas. It's not all about the time it takes to get somewhere, but the time I need to be active during it. My rail network lets me flick a few leavers and jump in a cart, then I can tab out and come back being exactly where I want to be. Portals require me to run about a bit before I get there adding unneeded hassle (I really dislike hassle).
EDIT: Also, new patch 1.5 just happened at me. Go go download it.