For a while, it was good and well, but, apparently, either people started racking up text messaging bills or just were afraid of doing so, and SMS support got trimmed - the message length couldn't be arbitrary or close to it (as it was in the beginning), and the number of messages sent to the other party before the reply was necessary to continue dropped - eventually to one, or so it appears.
And then the denial of service came into effect. Consider the following scenario:
- You desperately need to reach someone;
- They're not online, or outside of data coverage area, so you can't IM them;
- You invoke the "switch to SMS" option and send them a message;
- SMS is not quite as user friendly for non-smart phones, and there may be situations where it is not practical (for example, you can easily read SMS while driving, but to send SMS is much more difficult than to make a voice call), so they either log in and IM you, or simply call you and take care of business;
- They conveniently forget (that is, assuming they ever knew) about the fact that they have to reply to you before you can send another message to them;
- You get stuck with a non-functional end of IM/SMS gateway - the counter doesn't expire, and if you try to "switch to SMS" a week or month later, the dreaded "X must reply before you can send more messages" error is still there.
Boom, you're dead.
Oh, and as soon as they remove your message they were supposed to reply to in order to reactivate your end of the SMS chat, everything is lost forever.
The only option left is to switch to the plain old SMS provided by your phone hardware - but that's much less convenient than using SMS gateway provided by Google (keyboard assisted, storable, searchable, etc.).
My guess is that expiring the switch counter in a reasonable amount of time would be fair enough.
Wonder how and when Google is going to address this.