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There are front shifters marketed for 2x10 or 3x10 speed setups.
Why should they be only used in combination with a 10 speed rear shifter? Do they have any relevant influence on the rear sprockets, like different gap sizes?
I would like to purchase a 2x10 or 3x10 front shifter and use it on my 3x7 bike, given there are no problems with this combination.
There will be a cosmetic difference—I'm sure the design of the front shifter will be different from that of the rear shifter. It's up to you to decide whether that matters.
A 10-speed chain is narrower than a 7-speed chain, and the cage of a 10-speed front derailleur is slightly narrower as well. I don't think this means that the cable pull for a 10-speed shifter is different (this is well-documented for rear derailleurs, not so for front). If the cable pull is different, you might have trouble shifting across all three sprockets.
