Update on this:

Per Lennox tech support (972-497-5799, BTW, and not an easy number to get) for this model furnace, a G26Q3/4-100-3:

1) That degree of corrosion is indeed indicative of a problem that should be addressed.

2) The furnace is shipped with the inducer drain plugged with a knockout, and opening and plumbing it is a optional field modification.

3) FWIW, Lennox did not believe that draining the inducer would solve the problem, which they felt was likely related to excessive condensation in the exhaust piping, a problem that needed to be fixed at its source.