Do You Always use the Yarn Specified in a Pattern?

Yarn: https://www.etsy.com/shop/emilyparson          Pattern:  http://www.ravelry.com/patterns/library/the-louise-laxton-shawlette

Do you prefer patterns to specify specific yarns or just a yarn weight?

I've been getting conflicting advice on this. I think it's because everyone has different paradigms in mind when they answer the question.  Some of my advisers tell me that the exact yarn must be specified and available.

Yarn distributors know patterns sell yarn so they think patterns should be written for the yarn.

Yarn store owners want patterns that sell yarn in standard weight DK, worsted etc. so they can move stock.

Yarn companies want patterns that sell their yarn. They often develop patterns specifically planned to make substitution difficult. They may develop yarn with non standard gauges or combine two yarns in a design to make unique projects.

Indie dyers use standard weights of yarn as their base because they can't afford to do a lot of pattern support. They want patterns written for standard weights. They often use free patterns and have a project knit in their yarn as a promotional sample. 

http://www.ravelry.com/patterns/library/the-prudence-crowley-vest

My best selling pattern (in the photo above) was done in a yarn that was discontinued the same week that I published the pattern. Since I've been designing that has happened to me several times. The last time it occurred, the yarn company discontinued the yarn three weeks after providing it to my client. The result for my business has been I am focusing on standard weight yarns because I want to sell patterns.

I choose the yarn for the photo at the top because of it's absolutely amazing colour. The irony is... the depth of colour is not truly reflected on a computer screen. I want to sell patterns so it's a standard weight, 4 ply fingering.

Yes, I do spend far too much time thinking about this kind of thing don't I?