A couple of years ago, both BB and Dimitroff (by then, GM of the Falcons) mentioned, independently, their changing philosophy about safeties - moving away from the "division of labor" style that uses one SS/LB in-the-box guy (Harrison) and one centerfielder, to two, or even three, interchangeable guys who can all cover well, blitz occasionally and help out in run defense (though not as their primary responsibility). Such a safety corps may take some pressure off OLBs wrt coverage responsibilities, allowing them to focus more on the pass rush and run-stopping.
The Pats haven't quite achieved that yet, necessarily. Meriweather is very good in coverage, but not much of a run defender. McGowan is great in-the-box but fatally screws up coverages. Sanders is merely adequate in both areas, but at least a solid (and therefore valuable) reserve. Chung seems okay in run defense, but it remains to be seen whether or not he can cover. Lockett is enthusiastic on special teams, but his from-scrimmage capabilities are completely unknown.
All this leaves plenty of room on the roster for another high-end coverage safety like Thomas. There are certainly other guys (Graham, Wilson) who I'd like to see fall to us at #22 and I believe there are some fairly decent coverage safeties likely to be available in later rounds, but I wouldn't be at all dismayed to see BB take Thomas at #22 if he falls that far (and my preferred guys don't).