I don't think the piston indicator made any difference to the likeliness for the door to split.
As for why they did it, only Hoover will know.
Based on my conjecture, I'd say Hoover put the piston indicator on the bag door initially as that was the "done thing" at the time with the Electrolux Contour and others, but when they did all the advertising with the U2462 and U2464 Autosense w/ light indicator side by side realised the U2462 looked too basic in comparison.
So they decided to fit an LED light indicator to replace the piston indicator on the bag door and put an emblem where it was so it didn't look plain.
I do believe there were also models manufactured for a short period of time which featured the new bag door but no light indicator and thus no indicator at all.
There aren't many around (if I'm right in saying they ever existed) these days, the reason probably being that people didn't know when to change the bag due to having no indicator to say so (of course the general consumer wouldn't think to physically check or bother to pay attention to suction levels or motor pitch in most cases) and as a result motors burnt out quickly.