I bought my polo bike with a biopace chainring. It's such a sketchy feeling ride without a derailleur, you really can't keep a fluid pedal motion because the chain binds at its tightest point, and goes completely slack at the loose. You mash, spin, mash, spin, smack your knee on the stem when your chain slips off, fix it, no bueno.

So yeah, get a chain tensioner.