Magic is all about pretending that you have a skill that you truly don't have. David Blaine can't really levitate, but is it wrong for him to pretend that he can? David practiced very hard to make it look like he could levitate, and he convinced people that he has an extraordinary skill. Because Mark Elsdon practiced very hard to make it look like he can do something that most people (maybe him, too) think is impossible, he convinced people that he has an extraordinary skill. Mark has practiced enough that no lay person can detect the trickery that he is using.
Levitating is an extraordinary skill, that's exactly the point. Nobody can actually levitate, so it must be magic. When you pretend to do a skill that people actually have, on the other hand, you are demeaning the skill by making it appear you've put a huge amount of time into something you actually have no clue about, and by making it appear that anyone who actually has that skill is just using trickery. Blindfold cubing is already something that looks impossible and we don't need magicians making that perception stronger.
Besides, I really don't think this trick requires a huge amount of practice. Adam Zamora has showed me a very similar trick, where he put on a standard competition blindfold and then solved a cube with a normal method. I couldn't tell that he could see - and he is NOT a trained magician, he did not take a lot of time to practice this stunt, he just noticed that it's not actually that hard to look under a blindfold. So no, I'm not really impressed by the practice Mark put in. There's a big difference between cool stuff like slight of hand or card manipulation, and easy tricks like looking under a blindfold. Any cube solver could do a pretend BLD solve within a couple of minutes if you showed them how... so I'm much more impressed by the basic LBL solve than the fake BLD solve.
Mark tries fake the impossible, not fake the possible. If everyone knew that with a little practice, anyone can do a BLD, Mark wouldn't be doing this trick.
I think it'd be better that way. (Someone should, like, tell him it's possible.) You have to realize that, as someone who actually can do a BLD solve [even though, as SK points out, I don't like doing it much], I'm really worried that people might honestly begin to believe that it really is impossible and only possible with trickery. The annoying thing is that Mark can do a LOT of damage. Think about it, he's a professional magician and he gives shows, and he's selling the trick to other magicians who give shows... I'm sure a lot more non-cubers will see this trick in a show (and come out thinking BLD is as impossible as, say, making the Statue of Liberty vanish) than ANY of us could show a live BLD solve to.