I understand that it's important to water sufficiently and not just dampen the surface.
But does it actually do damage to water very briefly, or is it better than nothing? I'm thinking about a situation where you might be rushing to work in the morning and your time is very limited.
This would probably depend on the nature of your soil, the type of plants involved, the stage in their development, their drought tolerance and whether it is long- or short-term watering.
In the case of seedlings, for instance, in very warm weather, short-term superficial watering cannot do any damage by causing surface rooting, since their roots haven't yet grown down very far, so a brief watering must be far better than none which would compromise their survival.
On the other hand, if, for example, we take a reasonably drought-resistant and well-established variety of sweet corn, growing in a humus-rich, water-retentive soil before 'cobbing', it might be better to withhold water for a few days, until you can water it thoroughly - unless, of course, we're talking about a 'one-off' or short-term watering. In fact, withholding water for a short time might even encourage the plants to root more deeply and extensively in search of it.
Mancuniensis' gave a good answer.
I'd add that you won't do any harm if it's right before harvest -- especially if you know the plants really need the water. (E.g. lettuce you're going to pick soon that needs a little watering.)
Also watch out for plants that are sensitive to changes in watering: tomatoes during fruiting (blossom end rot and/or cracking), cabbages with mature heads (splitting), etc. If you do a "little" watering on a regular basis (weekdays before you head to work), then flood them with water (a weekend soaking) you may injure the crop.
I invested in a hose timer and some soaker hoses so I can (a) give a good soaking and (b) provide a consistent amount of water to plants that are sensitive to regular water levels. (Tomato, cabbage, celery.)
The other answers seem to be good, from what I read. However, I just wanted to put in my two cents based on my experience.
A little water has never seemed to hurt any plant I've tried it on. It should help a little. How hard the sun is shining, and how clay-like your soil is, should influence whether it's worth it. If the sun isn't out, go ahead. If it is, it'll probably dry up fast, but I don't think it will hurt at all, whether or not it helps. If the water sinks through the soil easily, a little water should be better than if it doesn't.
You can always shade the plants after the small amount of water to make it last a little longer, but if they're sun-loving, I don't recommend doing this a lot.
I have found, however, that when using small amounts of water, my plants seem to prefer filtered water over the regular chlorinated city water. It seems to perk the plants up more with less water (at least the tomatoes I tried it on). However, we had leaves over the soil to keep it moist (although that increased our cutworm-like problem, I think). Those tomatoes didn't require much watering. They weren't in full sun, either, but they still produced well for how long they had been growing. They were a hard, pasty tomato that sliced well, grown from a store-bought one that wasn't pasty (marketed as either roma or baja, I believe).
Covering the ground with something safe for your plants may help to reduce the watering needs. Black plastic is somewhat helpful there, and helps to control weeds. Leaves may be good, but they may increase insects and such in the soil, as well as mold and whatever else.
