Except for two 8/4 x 3-1/2" pieces of Poplar (Maple probably would have been better of course) face-glued together along the front side of my workbench (where the dog-holes are drilled), that's exactly how my workbench is constructed, and it works fine. Just get some nice straight-grained tubafours, joint one face and one perpendicular edge, run the opposite face through the planer, cut some biscuit slots in the faces to help with alignment during glue-up and you're good to go. I'd make the boards longer than you need so you can cut the planer snipe off the ends prior to glue-up.
I used 16 yellow pine two-by-fours, which after jointing and planing yields about 21" of width, along with the two Poplar boards on front for a total of about 24". As I recall, I glued them up in stages, maybe four at a time, until I had two slabs of 8 boards each, then I ran each slab through the planer to get the top and bottom faces relatively true, then glued those two slabs up to make the final 21" slab. Some minor cleanup with the Bailey No. 7 hand plane got the top surface true enough for a nice beater workbench.
Obviously not as nice as a full-blown Maple or Beech workbench, but if you're like me the thing is going to get LOTS of abuse, so other than gaining some extra durability by using hardwood I don't really see the need. I've beat the crap out of this thing and it's held up just fine. Oh BTW, I DID make my tail vise and front vise faces out of Maple though; I don't think Pine would be a very good choice there.