I'm definitely of the opinion that CNE and Ontario Place are big failures. They've been designed like distant suburban fairgrounds with large parking lots and no consideration for integrating them with the rest of the city.
Try cycling from Harbourfront to Sunnyside beach and you'll see what I mean. It's impossible to get past Ontario Place without cycling on the road or else balancing on a ridiculously narrow, poorly-maintained and windy biking path and dodging cars and pedestrians.
There should be residential space right up to the edge of CNE grounds, and a few serious year-round draws, eg put a museum in the Better Living Centre. About half the buildings on the exhibition grounds seem to be empty for most of the year. Every time they change something on the CNE grounds the architecture becomes more of a hodge-podge and the space become more disconnected from the city and internally.
Given the location, and the beauty of existing buildings and gardens the CNE should be a year-round tourist draw, but it's very hard to get to and once you do there's nothing to do unless the Ex happens to be on.