1. Every situation you just described is an offensive to defense substitution or vice versa. You know, those times when they're normally off the field and aren't busy being all-pro.
2. Your analogy is no different than ET playing special teams, which he already does.
3. 3rd downs happen a lot more often than any of the situations you described.
4. Some of us prefer that we find a suitable nickel corner not named Marcus Trufant.
5. Some of us prefer our all-pro free roaming safety to be an all-pro free roaming safety.
How about when we moved Aaron Curry in at DT during 3rd & long? Cory Redding going from 4-3 DT to 4-3 DE to 3-4 DE? Sure, some of these moves were to get more out of a deficient player whether it be Redding not having the speed to play 4-3 DE, Curry being unspectacular at DT but better than our other options at the time. But what about when DEs like Jared Allen switch back and forth from LDE and RDE? LBs like JP and Orakpo moving from WLB to DE on passing downs? DBs like Revis and Carlos Rogers moving in to cover Welker in the slot? Not every player has a skill set limiting them to their natural position and I'm not willing to discount ETs overall coverage ability for the sake of being closed-minded.
If ET were to cover the slot, I'm not saying it would be ideal for him to do it full time. I'm saying we could look into him playing the slot for preparation against teams that feature a match-up more favorable for ET than our usual slot guy. The irony is you say some of us prefer that we find a suitable nickel corner not named Trufant as some sort of logical rhetoric and yet, guess what? ET isn't named Trufant.