Actually there is; but apparently not for optical *ethernet*. Optical fiber channel can allegedly negotiate 2 speeds down. Not that I've tried it. But, then again, those are different transceiver modules.
Not entirely true. On Nexus I'm using the 10G twinax running at 1G. I used the "service unsupported-transceiver" command on the interface and it came right up. So far no issues. I know this isnt fiber so it may not apply.
In my case this was a quick fix because no RJ45 1G SFP's were on hand but there were excess twinax cables available. We are ordering those now. Other than support issues, if they work I dont see why we shouldnt use them.