The trouble with the downtown Grimsby Station is that it is tiny. The platform is barely three cars' length. When the VIA/Amtrak train comes through, if anyone is getting on or off, they do so from a single door. It's certainly possible to extend the platform, and parking can be squeezed in around it (on the north side of the tracks, there's a minor hydro corridor), but that's not really GO Transit's thing. They like big stations with lots of parking. (It's amazing that they allowed a minimal, downtown-ish station at Acton).
But I believe that Metrolinx owns the Casablanca site, and it fits in to their suburban station strategy. Casablanca is not a horrible place for a station (there are worse stations in GO's network, including its two newest on the Richmond Hill line), but if it's going to be out there, it needs local transit access to it.
I'm also skeptical of the ridership of GO Trains out towards Niagara Falls. It's still faster to drive from Grimsby to Burlington and take the GO from there. Commuters would be better off with a one-shot bus to Burlington from the park and ride already there. The slow orders through Hamilton slow things down, as does the many stops as far as Oakville or Clarkson. It'd have to run on a different stopping pattern from GO local and express trains. Regional rail makes more sense. There might be some commuter traffic headed to Hamilton, but it's not a big office node, and West Harbour isn't well connected to HSR buses to McMaster (the A-line does at least link it to Mohawk).