It's called an insurance policy. So that you know if Smith or another doesn't win the job, you have someone that at least has some experience and can be competent with the job.
And no, they aren't expensive role players. Jacoby Jones only costs the Ravens $1.6 million in cap space this year ($4M of the $6.5M in his 2-yr. deal is next year's base salary + roster bonus). That's the same price that Will Svitek costs this team currently, and a little less than Peria Jerry ($1.95M), Harry Douglas ($2M), and Matt Bryant ($2M), and slightly more than Snelling ($1.06M) and Vance Walker ($1.26M). If Jones proved to be a competent to good return man, wouldn't that be worth it for one year. Obviously, whether he's worth committing $4.9M in cap space to in 2013 is a completely different question with a completely different answer (which is no, unless he proves to be a reliable deep threat/No. 3 WR).
But whether it's Jones, Parrish, Eddie Royal, Domenik Hixon, Michael Spurlock, Louis Rankin, Lorenzo Booker, Mewelde Moore, Steve Slaton, Patrick Cobbs, Yamon Figurs, Courtney Roby, Tiquan Underwood, or Johnnie Lee Higgins, IMO the Falcons should have kicked the tires on at least one of those names to come into camp and compete for the job. Ideally, you would whichever of those players that would play for the veteran minimum and no signing bonus (unlikely to be Jones or Royal in that regard)
Now if Smith or whoever runs away with the spot then good for them. And if that veteran player isn't better than the likes of Kevin Cone, James Rodgers or Kenny Stafford then you cut him. You've lost nothing in the equation. If he is better than them, then you've actually found a way to improve your roster.
But god forbid this team found some way to free up an extra $700,000 and leave Dawson Zimmerman without a job just to provide a little bit more peace of mind on special teams...