Join Date: Dec 2007
Location: Shippensburg, PA
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As others pointed out, the cable pull is different. You still may get shifting in the front, but the shifting performance would suffer.
The other thing no one mentioned is the chainline - MTB derailleurs target 48-50 mm, while the road doubles aim at 43.5, and the road triples - at 45 mm. The end result would be a sour front shifting.
This you would not be able to rectify, unless you find a way to offset your cranks.
Yes, there are ways (shim the chainrings on the spider, use BB with a spacer, or adjustable one) depending on what cranks you have.
The gist is, don't do it.