I would say Yes!

Like you say they offer some protection from the elements!

Regarding heat; They are not designed for heat as such, they are just designed to keep the temperature a little warmer than the outside temperature.

If the open ends are a problem get a pane/s of glass of a width and height to cover the opening and this will increase the warmth substantially and you can remove it/them easily for ventilation purposes when conditions get too warm.

In plant terms; this increase in temperature although small in human terms, can be quite substantial in plant terms.

Think of it this way; The current Global Warming/ Climate change issue is causing great concern across the world, and they are only talking in terms of temperatures of 5°-6°C so I am sure the few degree increase your cloches are producing won't be the end of the world! ;o))

Or, instead of glass at the ends, use a plastic bag and skewer it through down each vertical side with a stick and push the sticks into the ground (does that make sense?!).

Personally I think the tiny polytunnels are more hassle than they are worth, cloches are a different matter as you can lift them up.

I got rid of the plastic on my Haxnicks cloches, but kept the wire supports, covering them with new green mesh which is much better.

That could work Diane