It's a great way to increase the amount of money they're extracting from their best customers while simultaneously giving them an increased perception of value. Seriously, they're going to print money hats.
To me it's clear that it's about the total lifetime customer revenue rather than adding up monthly prices. Especially since they are trying to get people to use Highrise and Campfire, which are much stickier apps.
But there's also the factor that if users do bump into any of the limitations at lower levels, they upgrade the whole suite.
Plus, their winning price points would seem to be $20-$50 a month (where managers and showrunners will be okay paying out of their own pocket as a productivity tool) and $150-$250 a month (which can be expensed to a company account without big hoopla if you know your people actively use it). And they want to get accounts pushed from the former category to the latter (it especially helps to be offering a good deal at this time).
And finally, nobody likes to see multiple charges from 37s on their card. That's definitely a real issue.
Good move for both customers and 37signals, I think.
That, of course, completely wiped out single-products that couldn't be sold in a suite, e.g. WordPerfect. Once the market was clear of competitors, they then proceeded to slowly but surely increase the price of the suite (and the price of the standalone products along with it).