For marines the Data sheet is not as important but for more complex armies like nids it is definitely handy to have all the rules and stats on hand.
I played a 550 point game and no two units seem to have the same M value!
Hormagaunts move 8, and consolidate 6"
Carnifex move 7 and has a bonus damage mechanic when he charged
Termagaunts move a pedestrian 6 but had guns, which are on the data sheet (all three options)
Warriors had the weapons I had selected on the sheet and they moved 6 too
Genestealers moved 8 and could charge after running.

I was certain to forget some of that during a game (i didn't remember how far termagants moved for example, but it took but a second to double check)

Meanwhile on the other side of the field. All marines had M 6 (except jump pack)
The only rules to remember were camo cloaks give +2 cover and that jump packs can FLY. Oh and the Captain lets you reroll 1s. All much easier to keep in the head.

In our second game, the tyranids got first turn and were much more aggressive. I can't see how the marines could get more than 2 rounds of shooting before being swept up in combat. A few good run rolls and some nice terrain will make it very hard for the marines to avoid being eaten.