Really depends on the niche/market/type of business.
I agree with the idea, in the sense that most companies should have a flagship product/site which is constantly being updated , new features added, analyzed, etc... they may then build out a few complimentary products, but never neglect the main business.
However, I disagree with the idea with regards to those who prefer to build/scale-out networks of sites.
For example, if you build a network of blogs with the idea that each is to generate a single sale a month consistently after setup with no further work required (yes, it is very possible). You obviously want to scale that out as much as possible.
So, both methods, if done properly, can pay dividends
Note: with the second method, the reason it often fails for people, is they don't necessarily set it up so it can be consistent without further work and it tails off after a few months of neglect. However, if done intelligently and with the proper research and execution, it is possible to have these kinds of setups simply keep growing.
Example: you acquire a solid type-in domain that is not "trendy", but something generic where you don't expect the traffic to fluctuate much year to year. This is much more consistent, say, than registering a new domain for a hot topic, throwing up a blog, making a few posts, spamming some backlinks, then wondering why in a few months traffic has dropped to nothing and it no longer makes sales.