Frozen veg almost always has enough ice crystals to steam itself. You can jumpstart the process by poking holes in the top side of a bag of frozen veggies, then microwaving the whole bag 30 seconds at a time until done to taste. It works surprisingly well if you're in a hurry.
For "actual" steaming, the advantage over blanching is that the cooking process is a little gentler. Heat is transferred at a steady rate into the food, and very evenly throughout, meaning larger batches are easier to cook all at once to the same "doneness". There's no water motion attacking your veg, so you avoid wear and tear, which can be important as frozen veggies can be more fragile than fresh stuff (e.g. broccoli).
The only disadvantage would be that steaming is somewhat slower, which means with frozen veg the outer parts can be mush while the insides are still cold, common in thick-cut veggies like some kinds of frozen carrots, cauliflower, etc. You can get around this by microwaving as above, just until the veggies are almost (but not quite) thawed.
I've worked in restaurants whose frozen veg was done both ways. I've found the results better with steaming.