If you have last year's Milepost, there's no real need to buy a new one for this year. Just keep in mind some of the businesses will be closed, while others will be open that aren't listed.
The Milepost is very ad oriented -- some businesses prefer not to, or can't afford to, advertise so don't get much of a mention in the Milepost. Yet their services might be better than an advertisers.
The Milepost is great for knowing where all the little turnouts are - good for lunch stops or overnighting. It also lists decent fishing spots, history, etc. I buy a new one now and then to aid with our exploring BUT mostly for the digital version which is then stored on the laptop, which I also use for livetracking with the GPS.
I rarely look at the ads, other than to snicker at some of the exaggerated claims of service.