I too had a soft spot for the old numbering system (Ferrari 27 was always one of the most passion-filled seats on the grid), but in the grand scheme of things, I don't really care.
If a driver changes his helmet design every 5 minutes, then a number isn't going to do much for him.
There is the culture issue of certain numbers thought lucky and unlucky in certain countries, but I guess that wouldn't be too difficult to get around... so long as we don't have drivers complaining their career was doomed from the start because they were allocated the number nine!
As for the numbers on the engine covers, I don't care about that either, but I'd imagine teams and sponsors wouldn't be at all happy with that idea.