Plastic pallets are vastly superior in the long run to wooden pallets in terms of durability, and many companies have used them for almost two decades interchangeably with wooden pallets. One of the issues here is that once a pallet enters the supply chain, who knows if or when you'll get them back. When I first started working for the grocery chain, there were over 30 pallets of back inventory sitting in the warehouse (this is a very bad thing and I corrected it during my time with the company).
The main problem with wooden pallets is that they are largely made with inferior wood that can't stand up to the stresses applied to them for a long period of time. If any of the center slats break, there's no problem. If one of the ends break, then you're probably going to be cleaning up a warehouse floor from whatever was on the pallet. People are also more likely to walk away with a wooden pallet since they have plenty of utility and are very inexpensive.
That isn't to say that plastic pallets are without fault, even if they are virtually indestructible. Most plastic pallets are manufactured with a diamond plate pattern. That's nice and all, but it's still plastic and therefore, slippery. Place some frozen or refrigerated food on a pallet destined for a windy area and have fun cleaning bananas out of the back of the truck.
So the best of both worlds would be a plastic pallet with a non-slip coating on the top of the pallet and the feet.
My sister worked for local fast food and general retail chains (Taco Time & Target) and I was always amazed at the lack of palletization they used. Target especially.
Target sends several trucks a day to each store. Each truck is half-full, and half of that freight is simply "stevedoered" into the truck, not put on pallets. So emptying a truck is a several-hour process of moving individual boxes of merchandise out of a truck by hand. What pallets they do use are often poorly stacked (by retail grocery standards), not well wrapped, and often fall over in transport.
Wal-Mart, by comparison, looks to have embraced the same "CHEP-only" policy as Costco.
* Trailers are 9-10 feet high, so a fully palletized truck would (a) Have very tall unwieldy pallets, and you'd have to carefully order things so boxes on the bottom don't get crushed, (b) need racks or something similar to hold multiple tiers of normal-height pallets, or (c) be half-empty
* Even with the current process, getting boxes off the truck is not even close to the bottleneck. When I worked at Target, 2 people threw boxes onto rollers, 5-6 people sorted the boxes onto pallets organized by area of the store as they went by, 2 people rolled those pallets onto the sales floor and threw each box in front of the appropriate aisle, and ~20 people opened the boxes and stocked the shelves. The 2 people unloading the truck could always keep well ahead of the 20 on the sales floor.
* An alternate strategy might be to sort the boxes into pallets by store area at the distribution center rather than at the store. You might get some economies of scale out of doing this process for 100 stores at 1 distribution center, but it may still not be worth it to put all that needed floor space, latency, and congestion in what I imagine is already a very congested point in the supply chain. Better to distribute that part to the endpoints, since you already have 100k square feet per store to work with that's just sitting there while the store's closed.
Part of Target's madness might be in the extremely different kinds of merchandise they're selling, but most of those products are small, and Winn-Dixie solved the problem by sending these products in plastic totes.
The largest Taco Time is an international chain of restaurants, found in the US and Canada.
The smaller Taco Time, officially Taco Time Northwest is a franchise local to Washington State, largely the greater Seattle area, where I grew up.
Taco Time NW has generally higher quality food than most other fast food restaurants, and seems "higher end". I always really enjoyed going there, and when I moved I was surprised to find these other places named "Taco Time" that were really different.
 - http://en.wikipedia.org/wiki/Taco_Time
 - http://en.wikipedia.org/wiki/Taco_Time_Northwest
That said, anyone craving Taco Bell while in the Portland/Vancouver area would still be well-advised to check out a local Taco Time instead -- it's still better quality food. Or at least it was 10-15 years ago.
Taco Bell has trouble proving that up to 35% of its beef comes from cows: http://sanfrancisco.cbslocal.com/2011/01/24/lawsuit-filed-in...
Taco Time's ground beef isn't even frozen: http://www.tacotime.com/menu/
Actually, Taco Bell had no trouble at all proving that their beef met the standard, and the suit was dropped pretty quickly.