Dolby Noise Reduction: Relevant or Not?

In the case of using a Reel to Reel tape recorder, if I play a tape that is using Dolby and I do not have a Dolby decoder, what is the result?

What Dolby decoders are suitable for Audiophile use?

Using a Reel to Reel tape recorder, would Dolby be relevant if the material was recorded at 15 ips?

dolby adds a certail increase in db when recording with it...then when activated on playback, it subtracts that amount. Ideally, you need a decoder if the material was recorded in dolby. Any dolby decoder will work, presuming that it is from dolby labs.
Amount added is mostly above 5kHz so playback without Dolby will be a little bright and noisy. Idea of Dolby NR is to suppress tape noise.
For reel to reel machine properly set, bais eq 15 ips there is no need for noise reduction. However if you prefer to have noise reduction you need to look for the professional Dolby A units.