Decoctions were used for highly undermodified malt, because you needed it to get full conversion. Today's malts, even the ones referred to as "undermodified", are still much more modified than the undermodded malts of yesteryear. These days, you can mimic the effects of the maillard reaction by adding melanoidin malt. However, it's a great thing to experience the process regardless. What I like to do in many of my beers is do a mini-decoction at the end of the mash. I might get a few points of temp drop over 60 minutes of mashing, so around 55 minutes through, I'll pull a stiff 1/4 of the mash out of the tun and bring it to a boil for maybe 15 minutes. I'll add this back to the mash and it will usually bring the overall mash temp up to 155-160. This will convert any unconverted starches, typically---at least in my experience---and the 15 minutes of decoction will definitely produce a modicum of melanoidin compounds.
I've done a true "full" double decoction before, and it's a bit of a PITA. First off, if you ever do it, please remember that because you've boiled your grains, you've disintegrated/broken up more of the husks than usual, and thus you will almost always get a false positive on your iodine tests. I sat there on my final sacch rest for 90 minutes and still pulled positive iodine tests. I finally gave up, expecting terrible efficiency because I left unconverted starches in there. In the end, it was the opposite---my efficiency was in the mid 80's. I figured out later that it was just false positives on my iodine tests. So, keep that in mind.