There is no historic precedent for the first contests not being winner-take-all. This is a whole new ballgame and that is why people are, quite correctly, stating that someone may well get in late.
It won't be a new ballgame. Proportional delegates will only help those with money in this race. Right now, few have it. Once a frontrunner emerges, all the other candidates will send their delegates to that person and they will win. The only one who won't is probably Paul if he gets any. This concept of a failed first ballot is a long shot at best. Obummer/Clinton was a nasty race set up like the one proposed here for our Primary. How did that turn out, contested convention? No.
The only reason this whole concept is accepted as a possibility is because the chattering classes love talking about silly things. If you recall, the reason the GOP came up with this stupid proportional delegate rule for early states was to create the excitement that the Rats had with their '08 Primary. What would be the sense of them doing this unless they started selling us to stay tuned to the possibility of a contested convention.