I think also that while it is perfectly true that people wouldn't "come down on Katherine in a ton of bricks" because of wearing a lot of valuable jewellery, there are nuances which could negatively impact on their image which would be very apparent to the royal family as a whole and their advisers. The UK taxpayer pays just over £40m, or roughly 62p per person a year to support the royal family, which is not a lot in the scheme of things, but for an individual could easily represent a desperately needed public service which has been withdrawn for economic reasons - and then for Catherine to appear in a lot of bling, while no doubt appropriate for the occasion and not representing any investment by the public, could nevertheless give rise to the question of why they don't just sell off some of the diamonds which are rarely if ever seen, have little or no historical or iconic significance, and help the country that way. I can't imagine that the royal family would want to imperil their increased popularity which is riding on the Queen's diamond jubilee, by excessive contrasts of lifestyle with ordinary people. It is interesting, in this context, that Crown Princess Letizia of Spain apparently had a new tiara some time ago, of which only a centre motif has ever appeared in public, worn as a brooch. Given the state of Spain's economy, it isn't surprising it's been kept very low key, though there is no indication it is anything but a private gift made from private funds.
Crown Princess Mary was given a historic parure left to her husband, and had a wedding tiara from the Queen of Denmark; moreover, these were presented in a rather more affluent time and people are now used to see her wear them and there is no surprise or feeling of undue showiness. Her other jewellery is generally quite modest. Given Catherine's current style and what appears to be her taste, there would be a genuine sensation if she were to start liberating some of the stuff in the vaults at this particular time, at least in any quantity. I'm sure OctoberLily is right and that as time goes by, this is what will happen, but I think it will be done gradually.