This advice doesn't help most startups, who haven't raised big funding.
When you have your own space, you're completely free to roam. You no longer need consent/approval from anyone else and you're free to customize things as much as you want. When you share a space with someone, you always partially depend on them in some capacity. When you're only sharing with people in your own company it's much easier to deal.
Also, in a co-working space, you could be sharing the conference call set-ups, the conference rooms, you have to arrange splitting the electricity/water bills, etc. All in all, it just results in more headaches.