BB Mounted Brakes - Bad Idea or Terrible Idea?

I have seen a couple of these now, direct mount brakes that fit under the BB.
For some reason it was done on some old mtbs

It's also done on some of the more "aero" road frames

Is it a bad idea or a terrible idea?

My understanding is the modern direct mount brakes are either centre pull calipers or a slimmed down v-brake. Old ones were cantis. If I was to get one of these retrospectively added to a steel road frame what would the best to get?
Direct Mount Centre Pull, a V-Brake, Paul Racers or even just stick a normal caliper under there?
I'm sure there will be 100 reasons why this shouldn't be done, but I'm curious to know what they are

The under-stay u-brakes were located there because this area was stiffer than the traditional seat-stays. I guess that's why wish-bone rear stays appeared soon after,to stiffen up this area for the rear brakes.

From mtbr fourm

According to Ross (Mr Salsa), the chain stays offerred a much stiffer mounting and would not flex under heavy braking.... unfortunately tended to clog up with mud in the wetter climates... we all can't live in SoCal!

Although the bikes limit how Rad you can get off road, add into the equation rock strikes.

There was a booming aftermarket in brake boosters for canti's and V-brakes, this was a horse shoe shape metal plate to limit some of the flex in the seat stays which robbed power.

Then again chromed steel rims and the faintest whiff of moisture was more of a death hazzard and wore away the soles of many shoes in panic braking sessions down big hills :)