Escaping co2 pressure should lift the top and allow it to flow out from underneath the solid cap sides. The benefit to this I'm assuming will be when pressure is at its lowest the top will create a air tight seal. In theory.
Your bucket lid does not have to seal tightly to keep contaminates out. They can not fly, thus can not navigate up under the lid and over the lip of the bucket.
The only concern would be a large sudden temperature change causing the pressure inside the bucket to go much lower than the outside and sucking air into the bucket. This would only happen after fermentation had slowed.