For the business I work for it is massively essential. We are primarily an eBay/Amazon business with other new channels coming in such as the French cDiscount and Rakuten. We have our own website, but the sales that come through it are quite literally a trickle.
Something which has highlighted this point is that we have recently had our Amazon account suspended (not entirely our fault I might add) and sales have dropped of by a good amount. At least half. So we definitely rely on multi channel. But like it's been said here already, it's dependant on what your business is. If you have something that can survive on one channel then multi channel may not be needed or worth doing. I think ecommerce is a bit too diverse to give a yes or a no on this subject. But I can't wait to see some insights from other businesses. This could be an interesting thread
As far as my experience goes, or rather such of my clients, the most successful businesses nowadays rely on a number of diverse channels.
They'd have a web site where, naturally, they don't have to pay the commission to the marketplace, however they'd need to invest into promotion of the website in search engines (SEO, PPC, Social marketing, etc). They would sometimes have multiple websites, or a single site with multiple front ends - one for each country, for example, or one for each range of products - in case they trade multiple ranges.
They would normally have an eBay account, or rather multiple accounts with eBay. It could be because they list their products in different countries as well. Naturally, they'd use Amazon in the same way. Other marketplaces like CDiscount, Rakuten Play - sure, why not.
Then, there's mobile. Mobile-friendly website or a mobile app (and with the app we have iOS, Android, Windows).
Finally, there's sometimes the call centre where they take phone orders. And the showroom - so there's a need for Point of Sale solution.
Add an Accounting solution like Sage to it too - not a sales channel, but contains all the data, all the stock, prices, orders, etc.
It is indeed a set up that more and more of our clients move towards.
Several years ago our task would be to build an Ecommerce website, or design an Ebay account. Now, it is so much more than that - it is about building those blocks AND linking them all together. Designing systems so there's a master database where the customer can enter product description, manage price and stock levels and work with the orders, and so that the master DB gets automtically synced with ALL possible channels in the most efficient way.
What I see from our clients is that they can not rely on a particular channel anymore - there are pros and cons about every channel (website - needs funds for advertisement, lower transaction fees; Ebay / Amazon - no advertisement costs, higher transaction fees, etc) and they try to balance to develop their business in all directions.
What they say is that they want to deliver the "omni channel" experience to their own customers, i.e. when a product is the same across all channels, when user experience is somehow similar, when special offers could be used online or off-line, mobile or in the showroom, etc.
I believe in the future there will be even closer integration between the channels, and also expansion into the global market - so there will be more emphasis given to international shipping and payment methods.