We need a 2nd stringer with "real" experience albeit he may be avg. or slightly below avg. in ability. That's much better then going with a 3rd stringer or someone with absolutely no playing time experience at all, which chances are, would make 2-3x as many errors as someone who has previous starting experience. It is always preferable to have a backup with previous playing time. As of now, the market is limited to a few players.
Previous backup QBs with "starting" time include Lucas, Griese (former pro-bowler), Feeley etc. Berlin and Lemon haven't thrown a ball in the NFL. Maddox may be washing up towards the end of his career (but would still make good decisions) and Harrington never lived up to why he was #3 in the draft (if i'm not mistaken) but statistically....played like Fiedler / Frerotte with something around 68Tds to 62Ints or whatever it is (right around there).
For a mill a year....they're insurance well worth it as a backup to most teams that don't have a backup with any playing time at all or only 3rd string or practice squad talent.