The downside is that no single solution is best for all conditions. A thick, heavy oil that would be best for rainy, wet conditions is probably not best for dry dusty conditions which is why most folks have both types of lubes available. In any case, you can't really get a chain fully clean unless you remove it. If you remove it and use your one-step cleaner/lube stuff it to fully clean the chain it might actually be OK for some conditions but if it can really get in and degrease the chain it won't be thick enough to provide much protection on a rainy wet day. It will also be much more expensive than cleaning the chain with mineral spirits, followed by rinsing in soapy water to dissolve the mineral spirits, then rinsing in clean water and drying. If you do THAT...then lube the chain with either a thick oil for wet conditions or a thin teflon or other thin "oil" for dry conditions you'll save money in the long run and have a better lubed chain. All in one solutions are for those that want to use a simple Park type tool to just run a chain through but that will never get the internal areas as clean as they can be. But people often over-think this kind of stuff. If you use that stuff regularly it will work just fine even if it's not as good as removing the chain. With a master link it's easier to just take the chain off and clean it properly.