Having closely read the BIP20 proposal, I can see your point. As I see it, BIP 20 vs BIP 21 is about standardising on a representation of the "amount" field. BIP 20 proposes that "amount" can contain alternative representations, clearly defined, whereas BIP 21 requires the use of a single representation in decimal notation.

In my view, BIP 21 still wins since it reduces complexity for the end client both at the human and machine level.

On 30 January 2012 18:56, Luke-Jr <luke@dashjr.org> wrote:
On Monday, January 30, 2012 1:50:03 PM Gary Rowe wrote:
> Speaking on behalf of the MultiBit team (Jim's currently on holiday), we
> will not be supporting Tonal Bitcoins anytime soon. Therefore we back the
> BIP 21 proposal.

It is not correct to imply that BIP 20 requires Tonal Bitcoin support.
In fact, the exact opposite is true; it states that even if one unit (eg, TBC)
would be a more rational way to display a specified amount, clients should
still interpret it in the way that is deemed to be most intuitive to the user
(eg, BTC).