I'm new to DCS, but this is my take. I have Steam because if you want to play games on a PC, they are pretty hard to avoid. For DCS, I have the program installed in a directory apart from my Steam game files, that is, one DCS install is on Steam and one is a "standalone". It takes a little extra hard drive space, but I don't 100% trust Steam. Maybe multiplayer is better on Steam, I'm not sure. For a few gigabytes of hard drive space, I get it both ways.
Today, Steam has DCS modules at great prices. I buy them on Steam and they install to the Steam game. I'll download the same modules from the DCS website and use the Steam CD codes to install them. If Steam servers get hacked, which would not surprise me at all, I still have my DCS install with the standalone. Like many computer users these days, I use an SSD for my boot drive and programs and a large capacity spinner for storage so a few gigabytes is not much for peace of mind.