IMO, a fence will slow down and delay many would-be crossers. But I've had a long standing challenge to anyone here which hasn't been accepted. Give me $5000 and I'll find a way to post an email and post a thread to this forum from any country on the planet.
Nobody has a secure border that will keep someone out with the right connections or $$$.
Of course it wouldnt be a panacea. Ive heard people who dont support a fence claim the same amount would tunnel under, swim around, fly in, etc. Thats absurd. A fence, coupled with a reasonable guest worker program and decent enforcement could work. Of course my guest worker program wouldnt be for mexicans only Id include the whole world so we would really need a fence on the southern border because mexicans feel they deserve all of our immigration slots and dont much like to wait their turn. I dont see a program without a fence working. Now getting the "guest workers" to leave when their time is up is another thing.