They'd be at best OK during the rain, at worst dangerous during the heat. The law simply states that your vehicle must be equipped with appropriate tyres for the conditions so it covers summer and winter in one hit.
Winter tyres generally have a speed restiction on them, although nowadays it's often something like 190kph, which is probably faster then most people drive. They are pretty noisy at speed though and I believe they will wear out much faster than summer tyres. If you read the report I linked to a few posts back you'll see that some of the summer tyres out performed some of the winter tyres in the wet, probably because it was a little too warm for the winter tyres. In really warm conditions the difference may be much greater.
Basically, winter tyres are for winter, summer ones for summer, don't mess about penny pinching with your safety.
Hardly penny pinching. I was only wondering out loud.
I just spent 950 euros on a set of winters, the old ones were still legal (but not recommended) at 3mm.