You even point out the ease of backup with this scheme...
Maybe with new light clients coming out this will find a more receptive audience.
Keep an eye out for Armory
which is a client I've spent 6 months developing, and should be released (alpha) within the next month. Among the plethora of fantastic features is that it uses Type-2 deterministic wallets for everything, and it gives you an option to print a paper backup of the wallet when it is created. And because it's type-2, it also supports watching-only wallets, so that you can create your offline wallet, make a no-private-key copy of it, and use it to generate addresses and monitor transactions from an online computer.
Teaser: below is a screenshot of Armory's paper-backup dialog -- yes, it only needs to be backed up once (unless you manually import addresses)
At the moment, it's about as far from lightweight as it can get. But the current goal is proof-of-concept of the features, and I have a development plan to scale-down/fork a "lightweight" version, and eventually release a sister Android/iPhone app.
P.S. - It will be open-source (AGPL v3) and the code can be found here
. (trying to drum up some hype for it before the first release)