It can work as a poor man's resilience strategy, sure. Assuming that Gmail is setup to receive email for your domain as well. It will get a little clunky if it happens often since the "mailboxes" wouldn't be synced so messages going to Gmail would be on their servers and mailboxes, and the users would have to login to their "mycompany" mailboxes on Gmail to even see the new emails...which would mean in essence you'll have 2 different mailboxes per user with most of their mail in Exchange and some "failover" mail in Gmail.
Backup MX records of a higher metric are supposed to be used after the lowest metric. However, there's nothing preventing a sending mail server from sending mail directly to any of your MX servers. In fact, some spam/mass mailers often will use a higher metric MX target because it can often lack the same spam filtering power (an example would be companies that set their primary MX record for a spam appliance with a "backup" MX record of their Exchange server in case the spam appliance is offline).
You also run into the issue of "what does unavailable mean?". If the Exchange server is completely offline that's one thing. But if it isn't and is just having "issues" sending servers may still try to connect to that server regardless.
Google Apps (postini) does/did have some continuity features built-in that could be incorporated with Exchange though. If I remember right they had 2 different options, one for "mail queuing" in case Exchange was down, and when it came back up it would deliver it on to the Exchange server...and then actual continuity where you could dual deliver and if the Exchange server was down you could still get your email in Gmail.