It depends! There must be some onsite work done. But who goes and who doesn't is a factor of skill sets, experience, the company, the number of sites, the size of the site, the uniqueness of the site, the experience of the employees of the company, the level of buy-in from the site, is it a new implementation or a major upgrade, is the site on an existing ERP platform, what industry is it, how similar are plant sites to each other, what are the projects' staffing level, what are the site staffing levels, what is the management structure, etc. etc....
Site visits are not just about collecting hard system requirements and raw data. A lot is breaking down resistance to the project, building support, allaying fears, listening to the various site resources, making sure their concerns are heard and addressed. It is about learning about the process and how things are done and how things are made.
Now if you are talking about the actual go-live implementation... again, it depends! Many of the same things mentioned above apply. I believe that a strong on-site presence at go-live is important. That might not be possible if there are a lot of sites going live at once. But you'll still have an on-site presence in some way at most sites.
Does one senior consultant really have enough cross-functional knowledge to gather all the info for one site? Finance? Manufacturing? Warehouse? Sales? etc... Very doubtful. But of course "IT DEPENDS!"
I don't think that probably helped you! 
Craig