Why don't you use eccentric bottom brackets or sliding dropouts for your single speed frames?
Each type of chain slack reduction technology has its pros and cons. We prefer the mechanical simplicity of horizontal dropouts. We do this for a few reasons, but mainly because the interface between wheel and frame has no moving parts and no added complications. Eccentrics and sliding dropouts are more prone to issues like creaking and slipping. The biggest obstacle people find to using dropouts like ours is that they do take a step or two extra to remove a rear wheel. We're ok with that. Carry a 5mm hex wrench.