I remember DHH talking about this on a podcast once i.e. for most super-successful companies, the lion's share of their of their PROFITS (not usage or revenue w/o profit) have typically come from 1 or 2 core products.
Think of Google and AdWords, Microsoft and Windows/Office, Amazon and their e-commerce business, etc. These companies all have lots of other successful products, each of which is used by millions of people and may generate lots of revenue, but at the end of the day their actual profits come from their 1 core product.
Apple is the most recent example of a company that has successfully been able to create multiple different product lines that were extremely profitable on their own.
So saying that Basecamp is 37Signals' "one good idea" is not a knock on them at all. It simply puts them in the same league as any other super-successful company.
P.S. DHH is an unabashed capitalist and he's gone on the record many times saying that he'd rather be part of a company valued at $100 million that takes home $10's of millions home in profit every year vs a company valued at $1 billion that takes home minimal or no profits every year (e.g. Groupon). I'm assuming that this sentiment is shared by others at 37Signals as well.