Yes its because nobody uses the promise, therefore it could be removed without any notice.
The promise was required in the history to assert that the trustee did not "produce" more "notes" than gold/silver. They knew that if there was any doubt in the value of the note, they could simply go to the trustee and exchange the note back to gold/silver, and if there would be more notes than gold/silver, the trustee would lose reputation very fast and nobody would trust those notes anymore.

Basically a IOU translation between gold/silver and money.
Basically, the promise would be used by those that had any doubts on the values of the notes. Since everyone trust the notes today (as opposed to history), theres no need for a bearer promise backend.

Currently, instead, people trust the state and government, and thus the money does have value since the state/government produces money with a specified rate, and thus, people find value in money since they are rare. In other words, the bearer promise does not have any value today. In the same way, Bitcoin is a democratic system where all the partipicants of the bitcoin network regulate the creation rate of "notes" collectively.
Even tought the bank would not satisfy the promise, you could always go to a jewellry shop to get your notes translated into gold or silver.
Same with the digits stored in a bank's computer. Those digits have value since the state/government regulates the system so no bank could create "fake" money. If they would, they would lose the bank license.

2 points to make

1. ever since the minimum wage came in. it became widely known that one hours minimum wage is worth £6.20 .. and a mirror to that obviously is that £6.20 is worth 1 hours minimum wage. put it into context of other things. EG a loaf of bread or 4 pints of milk are worth 10 minutes of minimal labour each.

now on that scale the promise would be that you will be paid in something in the equivalent to just over 1 hour 30 minutes of minimal labour in exchange for a £10 note if the bank note disapeared and the government had to trade your bank notes for something new/different.

now lets move onto bitcoin. although its harder to judge true value. i have seen the network hash rate of mining and seen similar waves as the network hashrate sky rocks so has the price. and where the price sky rockets totally unrelated to network hash rate thats where things like the 2011 spike and dump happens.

as long as you can see some form of correlation between new users buying into bitcoin, network hashrate. then bitcoin has a value. if you cannot see the price follow any pattern or statistic at all, then its a speculated bubble.

bitcoin is still too new to have one single set guideline for its value.

point 2.
FSA have no issues of you making bearer bonds and promisary notes. they do however have issues with reproducing the UK pound note using the word 'pound' the symbol '£' and the queens face all in one place without their consent or explanation. but so as long as you ask FSA and the UK government you can (with alot of paperwork and red tape) make your own pound note.
take these guys for instance.
http://bristolpound.org/

No.
Thats what many shops sells today. "gift cards" are they called, and for the gift card, they promise to give you Products of value in return.
So essentially, gift cards are "bearer bonds".

However, the legisation around ANYTHING value-filled (even a metro card), is pretty harsh in EU, and you need to Know your customer and be viligant against Money laundering and financying of terrorism.