It's a nice addition for flexibility. Another way to explain the limitations on it, you can't end up with more than +2 or -2 after adjustments at any one step of character creation. (So the sickly dwarf above inherently gets +2 CON, then they apply their floating flaw so they are, so far, net 0 for this step, then apply one of their 2 extra flaws to end up -2 for this step of character generation.)
Yes, this 'loses' you stats as compared to someone else's array who did not choose this, but they couldn't make it NOT cost something or everyone'd be doing it. :)
