I'd say it is multidimensional problem. It's true there is a very high number of unskilled workers that are the ones that were left behind after the construction bubble. Their skills are in construction mainly and it's difficult for them to find a job.
But the low wages in skilled job are a prebubble problem. In fact I think they are due to a "titulitis". People (by traditional family request) needs to have a university tittle, and no real expert technician studies (really skilled electrician, plumbers, woodworkers, etc..) like in other countries like Germany were they are well respected and with high wages.
In fact the entry wage to construction worker was maybe double to the one any university graduate had. It was not uncommon for a construction unskilled worker to make in excess of 3000€/month while a fresh law or economy graduate with English and a Master couldn't find anything better than 1000€/month. Quite a shame and an inverted market.
Of course now with the bubble over, the 3000€ wages in construction are over, but the 1000€ wage for people with a title remains.
It's true that Spanish company owners don't expect that someone with a title will be able to add value behind the 1000€. I've been thinking why for quite sometime, one of the reasons is that most Spanish entrepreneurs used to be self-made, rising their business from the bottom with little formal education. So they didn't find that that education was truly valuable. Also the oversupply of graduates already before the bubble, made finding someone for the job easy. Another reason is that most Spanish people can rely on the family network so it was very rare to see someone moving to another city to find a Job, they rather stick to a low pay and stay close to family and friends. This has changed with the crisis, with people people with studies or experience emigrating to the EU and south America mainly.
I do think that Spanish people that goes abroad to work is mostly as good as the other European or American counterparts, hard-working and well prepared. But it's also true in Spain that a lot of people that is not interested in moving, are also not interested in making an effort in their jobs. I can say this of myself, I work as an airline pilot, and certainly wages are much higher in the middle east or asia. But I'm not interested in earning more just to be stranded in the middle of the dessert or in a megalopolis. I'd rather stay here where quality of life is very high even with a lower wage (family, friends, nature, culture, etc... are just 10 min away with awesome food and nightlife). For example one of my best friends is what you'll call a 10x programmer, is the star programmer and architect in a tour operator, he has built mostly alone and from scratch the core of a hotel reservation company with sales figures close to one billion €. He is the one who is called to put down fires or to handle difficult projects. He is very happy making around 100k€, when I'm quite sure he could be making 5x that in SV, and he is the best paid programmer in his company.
I know a number of business owners that complain of being unable to find people willing to do the work, it's true that the initial wage will not be very high, but usually are very specific jobs that need learning. And this business are interested in paying better once they have the skills. This includes electricians, beauty parlour worker, car mechanic, boat mechanic (I´m talking just positions I´ve learned are needed this last year). They find that workers are not interested in learning or working hard, just ask for what is the payment the first day, and they don't even know how to do the job!. Even people desperate are not willing to make the effort to learn a new skill or hard work, because most of them have the family that supports them. Even my friend complains that a high percent of his programmers are not interested in improving or learning new things.
In Spain most people want to be civil servants, entrepreneurship is something that just recently has started receiving some recognition. Before entrepreneurs were seen as just bosses that want to take profit of people, some kind of villain. That's why I'm not very bullish of the universal wage as an "universal" thing. You see that a high percentage of people is not interested in working any more once they are able to have enough money to live. We have this institutional envy in Spain were successful people (business or professionals) are seen as cheaters no matter what. You can not make money working hard, just cheating or stealing, so people don't even try. Don't take me wrong, there is very hard working people here, but they do it as their responsibility, because they like their job well done, even when they are poorly paid. Usually no as a way to improve their conditions or earn more.
In Spain living from the public money is for some like an art. Working in "b" jobs that are not registered so they can maintain the government help, or working just enough to have the next 6 months of unemployment, or trying to obtain a health disability (when they are fine). With the crisis all this went down, but still...
You also have to think that that as we had the Euro and we couldn't devalue the currency to keep up with the crisis, they had to devalue the whole country, taking most wages and prices a 15 to 20% down over 6 years. This has been very very painful to the economy, but I don't see they had other way to save the Spanish economy.
I must say that my experience is in Mallorca island and Madrid where the crisis has been melower than in the rest of Spain due to the tourism for example. There are towns in the middle of the peninsula, where unemployment is dramatic an nothing has come to replace construction even after all this time.
It's a complex problem both at the supply and the demand side. Companies don't value properly good workers, and also workers don't see their job as something to nurture, but as an inconvenience and the management as oppressors.