Because sometimes Bitcoin or number of Satoshis is more appropriate. It all depend on specific tansaction whether Satoshis, mBTC or Bitcoins makes most sence to visualize in my brain when doing the transaction

It is depended on people preferences, personally i hated to deal with decimals,
But i am familiar now, and i like more to deal with decimals instead of satoshis or mBTC

The best is that all clients let you switch quickly to the unit you need at that moment.

in my opinion.. i think it doesnt matter if its mbtc, satoshi or btc.
as all bitcoin users understands already what is it.
but when you tell that we start using ONLy mbtc. there will be some
debate over it. as some people will like instead change it to satoshi only. or btc only.
so i think theres no need to change the usual one. you can measure it in mbtc if you want.
but i doubt that everyone will agree using the same also for over all.

Maybe "bits" will be better, as we expect the value to increase significantly? At the moment 20 bits is about 1 Japanese Yen. Maybe soon it will be equivalent?

1 BTC = 1 000 000 bits. easy.

i like to use more satoshi as the minimal divisor, i simply say i send you 0.01, 0.001 etc...

i don't have problem with rational numbers

Agree with you we should start using mBTC instead quoting BTC.I think there less awareness about this even we see it everyday.I will start using from now onwards.I think is good initiative to debate and spread the word out.

Im fine with using Numbers like 0.001
No need to change anything.. but use whatever you want.