Although the local train to Centraal is included in your Eurostar ticket, there are other factors to consider. The train transfer isn't hard, but since the fastest of the IC alone trips (to Centraal) is like 2:22, a 1:55 Eurostar that leaves you at Midi when you want to be at Centraal doesn't seem like such a good deal. There are MANY local trains per hour, but besides the latency (waiting for the next one), you have to consider waiting for the one elevator if you don't want to carry your bags down the stairs to do the change. And the local trains tend not to have luggage racks, but you can just stand in the large vestibule for the very, very short ride, if the train happens to be crowded.
It's also significant that many low-priced Eurostar tickets can't be changed for free, if you miss the one you booked. The conventional (i.e. "slow") trains tend to be completely flexible on the date of travel.
https://www.nsinternational.com/en