Unfortunately, many pilots think that, with two engines, you have redundancy. Truth is: maybe yes, maybe not....
A multi engine airplane which can't sustain flight, or maintain altitude with one engine failed, is still a redundant aircraft; engine failure is only one form of failure. More common is a vacum pump failure, and a light twin with two vacum pumps has far more chance of not suffering an insidious decay into partial panel than a single...or an electrical failure or a hydraulic failure.
A single, by comparison, is absolutey bound for mother earth after failure of it's one and only engine. The light twin may be headed the same direction, but with more options, a decreased descent rate, etc. Further, for IFR operations, the light twin is generally far more redundant and safer in terms of handling potential systems failures.
The second engine was never there to prevent a descent with one failed. The second engine is there for added performance when both are operating.
That the aircraft may not be able to maintain level flight with an engine failed is really irrelevant, making it no different than the single in that respect. Certainly it's redundant.
Any pilot which has received adequate instruction will certainly not believe the airplane will fly with an engine failed under all conditions. If a pilot has not received adequate preparation and doesn't take the time to calculate performance, then no amount of repeating the obvious will help him or her.