There is a trivial attack: a straightforward double-spend with shares you bought or hijacked. But, of course, nobody will wait only 10 seconds to confirm payment which has value above $3000. So it is not very practical.
Not only is it not practical, you must also control the SH for the exact 10 second period you intend to fool someone, and you must also be fairly sure you control his view of the network because Decrits will not just drop a second TB for the same time frame. It will be used as proof to destroy the share.
However, I don't see how this system can survive network splits. Don't forget that shareholders can pretend that there was a network splits when there was no...
The notion of a network split is pretty silly. I know Satoshi thought this was a real problem, but you aren't going to fool the world. If a nation shuts down its internet, everyone knows. If half the SHs disappear without a word, everyone knows. Regardless, resolving this issue is still not a major problem, just a trickier one. I haven't decided on an exact scenario to resolve legitimate network splits, but only because it has never been a priority. I will get to it when I get to it.
I believe Byzantine fault tolerant synchronization (it was mentioned by Ukigo) is a better approach... Basically you need to aim to get 50+% confirmation as soon as possible.
It is not a better approach, that was an earlier design that I dropped. If the notion is that everyone has to wait for 50%, then everyone has to wait for at least half a day or however short you are forced to make consensus. It is not a bandwidth-friendly nor time-friendly approach. The TB chain system works better and is more tolerant of missing blocks or blocks slightly out of order.
If there is a network split it won't be possible to achieve consensus, thus users will have a clear indication of whether payment they are receiving is confirmed is not.
Users will have a clear indication of whether payment is confirmed or not by there being a missing TB. If the chain is relatively unbroken, you know there is no network split. If it has started breaking up recently, a network "split" or "takeover" might be happening. 50% confirmation is only useful if you expect devastating network splits to be a common occurrence. It's not the efficient way to do it.