Evidence:
People are greedy. When people have the opportunity to exploit others, they often take it.
If anyone gets a hold of your em, they can torture your for subject aeons. Anyone who has a copy of your em can blackmail you: "Give me 99% of your property. For every minute you delay, I will torture your ems for a million subjective years."
And what if someone actually wants to hurt you, instead of just exploit you? You and your romantic partner get in a fight. In a fit of passion, she leaves with a copy of your em. By the time the police find her the next day, you've been tortured for a subjective period of time longer than the universe.
Very few, perhaps no one, will have the engineering skill to upload a copy of themselves without someone else's assistance. When you're dead and Apple is uploading your iEm, you're trusting Apple not to abuse you. Is anyone worthy of that trust? And even if you're uploaded safely, how will you store backup copies? And how will you protect yourself against hackers?
Sound more plausible now?
If you postulate ems that can run a million subjective years a minute (which is not at all scientifically plausible), the mainline copies can do that as well, which means talking about wall clock time at all is misleading; the new subjective timescale is the appropriate one to use across the board.
As for the rest, people are just as greedy today as they will be in the future. Organized criminals could torture you until you agree to sign over your property to them. Your girlfriend could pour petrol over you and set you on fire while you're asleep. If you si...
A new paper has gone up in the November 2011 JET: "Ray Kurzweil and Uploading: Just Say No!" (videos) by Nick Agar (Wikipedia); abstract:
The argument is a variant of Pascal's wager he calls Searle's wager. As far as I can tell, the paper contains mostly ideas he has already written on in his book; from Michael Hauskeller's review of Agar's Humanity's End: Why We Should Reject Radical Enhancement
John Danaher (User:JohnD) examines the wager, as expressed in the book, further in 2 blog posts:
After laying out what seems to be Agar's argument, Danaher constructs the game-theoretic tree and continues the criticism above:
One point is worth noting: the asymmetry of uploading with cryonics is deliberate. There is nothing in cryonics which renders it different from Searle's wager with 'destructive uploading', because one can always commit suicide and then be cryopreserved (symmetrical with committing suicide and then being destructively scanned / committing suicide by being destructively scanned). The asymmetry exists as a matter of policy: the cryonics organizations refuse to take suicides.
Overall, I agree with the 2 quoted people; there is a small intrinsic philosophical risk to uploading as well as the obvious practical risk that it won't work, and this means uploading does not strictly dominate life-extension or other actions. But this is not a controversial point and has already in practice been embraced by cryonicists in their analogous way (and we can expect any uploading to be either non-destructive or post-mortem), and to the extent that Agar thinks that this is a large or overwhelming disadvantage for uploading ("It is unlikely to be rational to make an electronic copy of yourself and destroy your original biological brain and body."), he is incorrect.