If I was going to late reg, I would then be taking the rebuy and add-on to give me 40,000.
I guess the benefit of playing from the start is that you can try to maximise the value of the initial entry and obviate the £30 rebuy (if you manage to accumulate some chips rather than lose them as I usually do). Of course, you should always be looking to add-on in these mega sats, the chips you get for the add-on dramatically increase the chance of winning a £330 seat compared to the £30 cost of the add-on.
I think the optimum strategy for these megasats is to play from the start, try to accumulate a stack, and rebuy if you can't, and then add-on. You're looking to get to the freezeout period with more than 40K, which puts you in a decent position to play the freezeout period. Of course, you can win a seat from far fewer chips, but it's more comfortable with a stack of 40K+ and you increase your chances a fair bit. It looks like a lot of people look to play without a rebuy, or manage to get through their rebuy and still lose all their chips before the end of the rebuy period - as quite a few of the runners who started the comp last night were out before the add-on. Obviously, these players would have done better to late-reg and go from there based on just last night, but if you do this regularly then you're forfeiting the chance to obtain more chips during the rebuy period.
People probably approach the sats very differently to me, and no doubt there'll be huge flaws in my thinking. All I know is that from a relatively small sample-size, I've been doing well from these sats over the past few years. Will be interesting to hear other people's thoughts.
(Apologies to Rich for hijacking his diary

)