I also opened a bunch of the other ones. Here is what I found (YMMV). Out of the 84, 30 were purple. Even though CWC has a small pool, there were few repeats (I want to say between 10 and 15, but I confess to not paying much attention to that - my brain shut down after a while because opening those envelopes is very boring).
Is it worth it?
For me, not really. Out of the 84, 58 were already known either because I found/bought them before this PTS cycle or I got repeats from the envelopes.
If you have very few CWC furnishing plans, I say go for it. Like I said, I got a small number of repeats out of the envelopes. As a side note, I'd recommend buying any of the envelopes of styles you have little plans. Again, the number of repeats was small across all of the envelopes.
I don't think the envelopes were intended as a primary means of obtaining furnishing plans, but OTOH, the more plans you know, the bigger the chance of getting something you already have.
Do I think the introduction of new plans will ruin the market?
No, I don't. Take for instance Morrowind plans, those envelopes have been available for more than a year now (I don't remember when they were introduced), but the price for them is still decent. The people who buy envelopes with the intent of selling their content will continue to do so, but will now have a larger number of plans to work with, which will prevent the market from being flooded with a handful of plans and causes the price of those to drop.
Master writ vouchers are a very limited currency, and the only one (IIRC) dependent on RNG to be acquired. Relying on RNG twice - to get the currency and to get a "good" plan out of the envelope - isn't a sound business plan. There are better, more reliable ways to have profit from writ vouchers.