I don't think SxS _absolutely_ required 1+, I think there were other methods as well, such as seeing if the second digit was 0 or 1 and routing the call. I'd have to check the Bell System history to be sure. In 1970 there still were a number of dial offices that did not offer DDD.
Some SxS offices got a "poor man's common control" front-end in the
1960s amd 1970s where the phone number was stored, translated, then fed out to the steppers. This allowed more efficient use of the steppers and trunking. It also allowed for DDD, Touch Tone, etc. The Bell Labs Record had various articles in the 1970s of various add-ons (some simple) to SxS offices to add to their lifespan and operating efficiency.Some panel/crossbar areas had it, such as 1+7D within the NPA for toll calls.
Message units were an easy way to automate a high volume of short toll calls that in other places would simply be toll. Phone bills in metro areas would be long and complex if every message unit call was itemized. It also allowed charges less than a nickel. If you look at rates at communities without message units, you'll find calls for similar distances had similar charges, only they were billed in money amounts.
As an aside, while a descendant of the message unit system is still used in the Phila metro area, the units have been reduced, some calls that were timed are now free.