You're right, but you're talking about an 'off the cuff', 'spur of the moment' ('guest singer') need to transpose a tune that you already know. The biggest difficulty (for me) is taking a solo (in the 'uncomfortable' key. The way I normally handle it (NOT spur of the moment) is to learn a 'spontaneous' solo ahead of time but this, of course, takes away from the spontaneity that is so fundamental to live jazz.

There's no debate here, really, just choices. Either use the transpose button or learn to be facile in every key. One is easy and one requires more work. One may be more personally gratifying to those that care about such things. On the other hand, the other may be seen as taking advantage of the advances in music technology. From the audience's point of view, it's the product that matters.

chas