He's good yea, but he's young so its a risk and who would he replace?......
On the bowlers: Yeah they're all fast medium but so what? Whilst one good quickie would be handy its not as though everyone else is stacked with them. SA have Steyn, Aus have Johnson, India have a decent (at this stage) one in Ishant, Pakistan don't because they're always either injured on on drugs. England don't, Bangladesh don't, SL have a liability in Fernando, WI have two variables in Edwards and Taylor.
What has let us down at test level time and time again is our batting.
Anyway, I have high hopes for Wagner, Davidson, Boult, Southee, O'Brien, Franklin. All bar Davidson will be in the minds of selectors right now (Davidson hasn't played enough yet, really like him, commentators liken him to Jimmy and his spell of 4/1 in last years final was fantastic)
Of course McClennaghan has bowled more accurately than ever, which shows he's learning. Still not convinced he won't end up an Amjad Khan (Englands new piechucker) though. The other quicks can't stay on the park long enough.
EDIT: If Butler was used right (short sharp spells) then I think he could make a good test bowler yet. Hasn't lost much pace at all.