It's a good idea in the short term (perhaps while construction on a new wall is taking place?), but deployed troops can just as easily be re-deployed elsewhere.
You have to break things down to understand what this means. He says 'if necessary.' Well, we've had a 'crisis at the southern border' for how long now? Seems as though it's been necessary for a while. He's always had the ability to deploy personnel, since the day he assumed office. It never really happened, until last summer, then again, just before the midterms.
If an emergency declaration was in the works, the deployment would consist of military personnel to build an actual physical barrier. No such announcement of a 'human wall' would be necessary. This seems like a trial balloon to gauge response to this in lieu of a real wall. To me, this reads of a feeling out move, because the rumors of McConnell warning him of pushback over the declaration might be true.
