Disney is not the only company to schedule "Grand Opening" celebrations months after the doors first open. Every grocery store, car dealership, and department store in your town does the SAME THING.
Opening up a grocery store or a multi-million dollar one-of-a-kind theme park attraction can be extremely unpredictable. Technical problems, contractor disputes, and other factors can potentially delay any project.
If you set your "Grand Opening" too early, you risk disappointing the public with an inferior product (Disney learned that from Test Track, Light Magic, and... believe it or not... the original Disneyland, which could have used a few more days of preparation. The public and the press SLAMMED the new park!)
Although Disney announced the October 9th M:S "Grand Opening" in April, that date was actually set in stone LONG before. Ad agencies had to be given time to roll out a campaign. Budgets had to be set. Dignitaries had to be invited/booked. Merchandise had to be designed. MANY departments needed a "Grand Opening" date to schedule their tasks.
Fortunately for us guests (and Disney), the company realized that M:S was way ahead of its extremely conservative construction schedule. They learned the attraction could be RELIABLY up and running by mid-August. That's why they made that awkward announcement of the "soft openings".
But even THAT date was too conservative. As we saw, people were riding M:S in mid-June.
The good news is that Disney has put all those extra days to good use. By now, cast members have had plenty of time to learn the complicated boarding procedures. Disney has already made complex changes to the pre-show, based on early guest reactions.
For guests, that means we got the chance to ride much earlier than expected (if I could go on Expedition:Everest today, I'd jump at the chance!)
For Disney, they can begin promoting the attraction earlier, hopefully boosting late-summer attendance, and certainly getting a jump on the slower Fall "off-season".
Would anyone REALLY want that building sitting empty--waiting for an official "Grand Opening"-- knowing the attraction was fully operational? I doubt guests would stand for it. Neither would Disney execs, who are anxious to see a return on their investment.
Some have speculated that this prolonged "opening" schedule will weaken and "water down" Disney's promotional efforts. I really doubt it. People are going to visit this attraction (and drop their money in Disney's coffers) whether it opened in June, August, or October.
Disney ads still feature Splash Mountain, 12 years after that "Grand Opening". And the sight of guests plunging down the hill is still drawing visitors!