While your statements are accurate in a general sense, they are a bit misleading. Pats 3-4 linemen do not directly translate into traditional 4-3 lineman. They absolutely do translate into the Pats 4-3 look (while still operating out of a 3-4 base).
Don't mean to poo-poo on your points since I'm inclined to agree with you (and said pretty much the same thing when I cut Santonio in the DL thread). I just heard back from folks that having a 7th DL is a good investment as injury protection. On my roster, there are 4 inactive spots for OL/DL backups. This has to do with the quality of the players as well as the difficulty finding big bodies on the street if disaster strikes.
That being said, I wouldn't be in the least bit surprised if the Pats enter the season with 6 DL. AD's ability to play a true end (not just rush LB at end) is useful as an insurance policy. Truth be told, I really like the rook LBs and would be plenty happy to use roster spots to keep as many as possible.