Interesting question and very relevant for today's world. While a large % of people talk about traditional vs digital marketing or will digital marketing replace conventional?, the truth is that marketing has become truly integrated. This integration is across various channels and thus the term multi-channel has become so important. While multi-channel marketing always existed, digital media such as Facebook, Twitter, Blogs, Videos have exploded the scope and opportunity.
At the same time, not every kind of organisation should look at all channels when they create their marketing strategy. For e.g., a startup is unlikely to leverage traditional channels. However, leveraging various channels within digital is still relevant and should be thought through for all kinds of organisations.
Now, the most important part: rather than starting from channels, one should start from the marketing objective(s), followed by target audience and then choose among relevant channels to create integrated marketing strategy.
Hope this helps. If you've a specific question related to multi-channel marketing strategy then please feel free to ask me here.