Yes crabapples can and are used to pollinate apple trees. The resulting fruit will always be the same variety as the mother tree and the seeds will actually be a hybrid of the two parents. Since we do not eat the seeds this works perfectly fine.

Many large scale commercial orchards have used crabapples in the past to pollinate large fields of a single apple variety . Many commercial apple tree sources still grow and offer some ornamental crabs for this purpose. The home grower can use them too for pollination if needed assuming the crabapple bloom overlaps enough with the apple variety you are growing for good pollination (usually they do).

Most crabs on the market are ornamental (have small fruit) but some really good larger fruited crabs do exist (Whitney, Centennial, Chestnut ect). My experience with large fruited crabapples is that they do not store well and get soft quickly. They can be excellent to eat fresh when picked at the peak of perfection. I canned many a jar of pickled whitney crabapples in my youth. The sweetest crab I have ever eaten is Trail which is yellow fruited. Very popular in the Dakotas and Northern Minnesota. Almost too sweet but definitely delicious.

My favorite crabapple is a red (actually pink) fleshed variety up to 2 inches long. It stores incredibly well, never gets mealy, is wonderfully crisp, and has a fantastic sweet-tart flavor. Oh yes, and it is very ornamental with nice pink blossoms and reddish foliage. It seems to produce exceptional crops every year, even when regular apples are in short supply. I'm still looking for an exact name match for this little guy...

Bottom line, crabs can be great!

And if you feel ambitious you can graft various apple scions onto crabapple trees. I had a Golden Delicious and Jonagold grafted onto Hopa Crab, an otherwise strictly ornamental variety.

Crab apples are the most tolerant of high county/northern latitude harsh winters than many "tastier" apples. They are great as
"pollinator magnets" to keep beneficial insects in the vicinity of other production crops.

My favorite crabapple is a red (actually pink) fleshed variety up to 2 inches long.

This is probably a hybrid between an orchard apple and a Rosy-bloom type of crab-apple (purple leaves and fruits, pink or red flowers).

http://www.raintreenursery.com/Red-Fleshed/

Other hybrids between orchard apples and crab-apples exist, for example the Dawson crab-apple, which is from a spontaneous crossing of orchard apples and the Pacific crab-apple. This is seen repeatedly in locations such as the Willamette Valley, a place where there are a lot of both kinds growing in proximity to one another.