I do this too. In addition to the previously mentioned problems with cheap switches losing their configs (and vlans) you should make sure the active interfaces are all on one switch so that the link between them isn't uselessly used; this will also avoid an unpleasant split brain event if that link ever happens to fail. But in this case you will also have to very carefully check the other switch stays properly configured so the backup interfaces will actually pass the traffic you want.
Linux's bonding module has an arp monitor which solves some of these problems, but the implementation is so hackish (as usual there...) that I'd rather not use it in production. arping and ifstated might do the same on openbsd, but I'm not sure if that will work when the interfaces are trunk ports. I'll need to check this when I have time.
