I don't think Tenard Jackson is technically on the roster,
He is on the roster. The team, knowing his situation, opted to keep him.
but even if he is he is still suspended for life from the NFL.
No he isn't. His suspension was "indefinite" because he had been in the NFL Substance Abuse program and had offended multiple times. An indefinite suspension is not "for life." It means that he has to comply with rules before being reinstated. Even Michael Vick--who served 2 years in Federal prison, and was suspended indefinitely--got reinstated. What makes you think Jackson will not be? I'd be curious if you have any precedent you're basing this on or if you're just being pessimistic.
Now most assume he will be reinstated a few days before the season starts, but what if that doesn't happen. Or he fails yet another drug test (definitely probable). He is most likely and upgrade over Madoodoo. I'll agree with that, but will he be on the team?
Most assume that because that's what generally happens. I can't recall a player who smoked himself out of the NFL and garnered a ban for life. Some guys have been in the program and just weren't worth bringing in. But, Jackson is talented. Shanahan likes him enough to have kept his rights through this suspension, and we are in need of someone with talent that plays his position. He fills a need, is talented and is cheap.
Dreading a CB in the 2nd or third round and expecting him to be an upgrade next year over DHall is bad odds. Vegas would side on that not working out and take every dollar they could get because the odds are it is easy money for them. Smoot at his best wasn't as good as DHalll was last year and he was a good draft pick. Yes. We could get a BC in those rounds that is an improvement over Hall, but the odds aren't very good.
I don't agree with your philosophy here. D Hall is a veteran playmaker who made the Pro Bowl. But, I don't recall winning any Superbowls with him, or any other team for that matter. We don't need a guy to be better than him in order to be an upgrade. If a young player can come in and play well within the system, and this secondary--with the return of Jackson and Merriweather, and an improved PASS RUSH with Carriker and Orakpo back, then the team can still improve.
How many years did we bring in talented guys who didn't play well, or play hard, or gel within the system? That's not how you build a winning team. We don't need to sign Namdi, Charles Woodson etc just to improve the secondary. We were the 30th ranked secondary last year WITH DHall. There's room for improvement, even if a rookie is in the rotation.