Cons of DD:
Insurance: I dunno if I could have even gotten it insured as a DD. If so I don't know what it would have cost. When I looked, I think both Hagarty and Grundy require the car isn't used for DD in order for Agreed value.
Repair time: If/when something does break on the car, it will take time to repair it. If you have NO other DD, you are down until its fixed. Car is 29+years old. Part's can still be found for stuff that breaks, but cost $$ and take time to arrive, even with Fedex. Can you be without your DD for 2-3 days in a best case scenario while you wait for a part, assuming you install it yourself and don't have to schedule it to be repaired. They are built using 1980's technology and rust is a real threat. Many will agree that when the car needs repairs, taking it to a modern repair facility (even a Buick dealer) is a BAD idea, unless the technician has first hand experience working on these.
Theft: Search these forums for 'Stolen'. These cars are one of the easiest to steal. If the car is your DD, you will have patterns of where you leave it and it is will attract thieves. Which explains the insurance argument above. You already will need multiple theft deterrents installed, but many times if the thief failed to steal your car, they will damage it enough to need repairs before being driven again. (See the above note about downtime for parts)
I think these cars 'can' be good for a DD one they are sorted out. When Turbo Regals were new, lots of people used them as DD's. I always seem to meet someone who DD's theirs. Some do it through winter. As other have mentioned, they can handle the job, are really comfortable to drive, and always turn heads.
When you buy one, you really don't know what you are getting into sometimes. Stock is increasing rare but means lot of things need to be replaced. 'Returned to Stock' is by far more common, but that means someone had a bunch of other stuff in there and removed it, usually just prior to the sale. Those Heavily modded cars are usually kept by either mad scientists or mechanical geniuses both of which had the motivation to 'See what this baby can do!'. You wont necessarily know which one built yours till after you bought it hehe. All you can do is ask questions, and request documentation.
DD just takes a toll on cars, even new ones. No way around it. As previously mentioned, most of us have a DD that takes the wear and tear because we want to enjoy the Turbo Regal. We usually only want to buy another one because we want more, not because we ruined the last one we had. Just looking around on CL and eBay you can see some that have not been kept with care. You usually can tell in teh pictures. Beyond the mechanical stuff that you can't see, their interiors with seats torn and carpet well worn, and the paint gets weathered. The paint is really bad on these cars, especially if it wasn't sheltered from the elements.
If you already have a DD and a TR would just be a DD because you have it, you can probably work it out if you were really proactive with maintenance, and had a solid guy to take it to for repairs. There are guys on this board who have TR's with 200k+ miles on the car and you wouldn't know it because they take THAT good care of it.
if you were planning on the TR being the only car in the stable, I really wouldn't recommend it, especially if you needed your car to get to work every day.
Here is a link to check out in case you haven't been there already
http://gnttype.org/general/buyguide.html
Good Luck in whatever you decide!