I think it all depends on your requirements, budgets, and on how much traffic you need to prepare for.
My community used to be structured like in your example, with separate (virtual) servers for each service, but not too long ago I ditched this approach and moved to one larger and more powerful server. I haven’t noticed any decrease in performance, and no one has complained so far either. It has made my life easier in regard to maintenance, as I only need to care for one server instead of a handful. It’s also much easier to set up from a security perspective.
Another point is that I don’t think Invision Community works too well with multiple web servers without some extra manual work. As you said, you would need to update the web servers one by one, but due to changes to the database all of them would have to be offline during the update.
I'm sure it's possible, but my community, and I think this applies to most other communities as well, is simply not large enough to justify the extra work and money it would require.