A study done by the University of Illinois in 1997 showed minimal nutrient loss in canned foods vs. fresh. So while the website you read seems to be accurate about the enzyme loss (all of what he said seemed to me to be common knowledge), if you aren't worried about enzymes, I don't think you should be too concerned. The major vitamins seem to be largely retained when canning, and you definitely want to destroy enzymes if you plan to preserve your juice.
http://www.pickyourown.org/PYO.php?URL=http%3A%2F%2Fnutrican.fshn.uiuc.edu/findings.html
That being said, I would argue that in general, enzymes are largely deficient in modern diets, and anything you can do on a daily basis to retain (by not pasteurizing) or restore (by culturing) enzyme/beneficial bacterial activity in your food is probably a good idea. We were once told that the enzymes in milk were unimportant, and that it didn't matter whether we pasteurized it or not, and I think that myth has been largely disproven. So, as others said, if you're going to preserve an excess of fruit/veggies at harvest time, steam juicing seems reasonable. If you're planning to juice fruits and veggies daily as some kind of "health drink," enzyme activity might be more important to you. But then you'll want to read up about whether you should be cooking your greens first, and whether they contain excessive quantities of oxalic acid - the trendy green juices drunk regularly may not be as innocent as they seem.
Do keep in mind that although bacteria have been found to survive at extreme temperatures/pressures, the bacteria in our food are NOT those particular bacteria.
And Eric Thompson, I appreciate your skepticism of marketing ploys, but if there is e-coli on your fruits and veggies that needs to be killed, I think you've got bigger problems than enzyme loss!
