This is in addition to specialisation, not instead of. Not picking on you, but I feel sales and other 'soft skills' are severely undervalued by many otherwise talented hackers. After a certain point of technical competency, the ROI of learning right brain skills is much higher. Getting better at negotiation could quite easily get a 20% rate spike.
Letting someone else worry about selling you, is how a developer worth $2000/day only makes $600. The person 'selling you', is also 'closing you' at the lowest they can get away with.
Being 'marketable' is basically a sales task as well. You may well be a specialist in some domain, but so are another 100 people claiming to be so. In big enterprises especially, the job most often goes to the person with the best 'story', not the most qualified.
It's like anything else; you have limited time, and you need to choose your focus. I personally feel there is a lot of 'low hanging fruit' in marketing for many technical people; I think it is pretty important to have at least basic sales/marketing skills, and that going from zero to just a little bit can give you pretty big (financial) returns without a huge amount of effort. A lot of this is to just get used to asking for more, rather than just taking what you are offered.
But it's also important to remember where your focus really is, and to not go too far down the marketing rabbit hole; at least in my experience, the ROI on effort in that direction drops pretty fast once you get past that initial low-hanging fruit, just due to my lack of innate ability, and at least for me, it quickly starts looking like deception. Really, much of sales looks like fraud to me, so I go out of my way to avoid those parts of sales. No sense in playing close to the line when I can't see the line. But, one can aggressive while still being transparent and honest, and while it does bother me that being aggressive gets me more money, it's something I can live with for more money.
I'd vote for specialization. Let someone else worry about networking and selling you; you worry about being marketable.