because of how a chain is put together, it is hard to find perfect chain tension with no extra tensioner device. sometimes though, you get lucky, and with the right combination of chain, sprockets and chainstay length, you might be able to get an acceptable chain tension without any extra devices. this doesn't happen too often, and if the chain stretches a bit, you're f**ked.
if the chain is too tight, you'll wear the chain and sprockets out quickly. if it is too loose, it will just come off the sprocket all the time.
stick with a tensioner. there are other places to save weight. a device similar to a surly singulator or dmr tensionseeker is not heavy, and should keep the chain tight enough.
i ride with a mech on the back though.. am actually running 5 working gears at the moment. you really don't notice MUCH difference with or without a mech as a tensioning device. i'd run one, or one of those expensive Rohloff tensioners, because they keep the chain tighter and safer than the dmr/surly tensioners, at least that's what i have experienced.
if you kill mechs/hangers all the time, take a look at the fresh products tensioner. it looks pretty sturdy, and should most likely work perfectly fine.