Dental Crowns for Teeth That Are Too Dark for Veneers?

Placing crowns instead of veneers to mask darker teeth is not necessary.  A well trained dentist working in conjunction with a good lab should have no problems masking darker teeth with veneers without the unnecessary tooth removal for crowns.

Nope! your dentist don't  have it right ...Sorry

You can mask dark teeth with venneers very well if he or she uses a good dental laboratory 

The root canal tooth definitely needs a crown though

Will add some photos later

There are new materials on the market that can mask a dark tooth with a veneer, for example E-Max.  It is beautiful, strong and could mask the color better than other materials.  If possible be as conservative as you can.  Perhaps some of the teeth could be veneered and the extremely discolored ones crowned to mask the darkness.

I do it all the time and get great results. The key is to use a talented enough ceramist so you get a great result without the staining shining through.

I would say you need to get a second opinion. A crown is used to restore lost tooth structure or reinforce a damaged tooth to keep it from damaging further. If the tooth is healthy and doesn't need reinforcing, I always go with a veneer to fix stain. The thinner the veneer the healthier the tooth will be.
I have masked severe tetracycline stained teeth with as little as 0.7mm of porcelain. It can be done, but the dentist and the team need to understand the materials. The "dead tooth" may or may not benefit from a crown. Because of the amount of reduction required for a crown, I would often still recomend a veneer.

Veneers will very adequately block out a dark tooth and blend with the existing teeth. Crowns require much more tooth reduction over veneers. There may be other reasons to crown the tooth, such as a previous root canal, decay on the sides or back of the tooth or large fillings. 

It is very common to do veneers on dark teeth. There are many techniques for masking out dark teeth, including opaque veneer material, blocking out tooth structure before veneers are placed, etc. It is likely that your dentist is not familiar with some of the methods or materials available. If this were MY situation, I would NOT be doing crowns, I would insist on veneers.  If your dentist says it won't work, seek a second opinion.
Dark teeth are one of the main reasons I place veneers, as do many others.