Well, almost nothing works perfect anywhere, but the welfare states of western and northern Europe (even more so 20 years ago than now), aren't bad examples of highly functional systems based on what I mentioned. To be clear, I'm not proposing a competely radical system like communism, or god forbid fascism (which actually is probably much more of a political than an economic system (although of course both terms are not alway cleary seperable), and most of the times the economic system in fascist states was basically capitalist). I do for example also believe that the characteristics of a market-based trade and exchange stystem are, in most areas, extremely useful to society.
However, if the economy of a society is in all areas completely left under the regulation of "the market", extreme inequalites occur that have nothing at all to do with meritocratic values and cannot be justified in any way using rationalist and non-religous arguments. Simply put, the rich will get richer and the poor will get poorer, up to a point were social mobility is reduced to an extreme minimum not unlike to feudal societies. What's more, this doesn't only harm lower classes, but after a certain point also the wealthy, which of course don't appreciate areas riddled with crime or large proportions of the society being only very poorly educated as well.
The notion that high taxes on capital gains, high income and the like in general, harm economic growth sounds nice and makes sense in theory, but just doesn't hold up to the facts. For example, the development of per capita GDP has been pretty much the same in all (highly) developed countries from 1980-2008, despite the massive differences concerning these taxations between let's say Sweden and the USA. Part of the reason for this is that low-end consumers having more money means them spending basically the entirety of this money, boosting demand (this might become a problem for a country if the addiotional consumption is focused on foreign goods - see Greece -, but normally, and at least globally seen benefits the economy hugely). Enterpreneurs and companies, of course not being stupid themselves, realise this potential of them being able to sell more of their products and invest accordingly. And assuming that you don't create a system of complete and total equality, which I by far am not proposing here, the number of people "leeching off the system" will be minimal and neglegible in the grand scheme of things. There are more than enough incentives for people to work and have success and in their professional lives even if they don't face living on the street without a job. In fact, the almost impossible difficulty of getting back into the economic system once you completely drop out of it (e.g. get homeless) is another reason why a strong social system is preferable, as it very often prevents this happening. Especially in times of short-term economic shocks and crisis, this can prove to be very effective in the long run, as this "dropping out" of mostly potentially useful components of the work force and decreased consumption represents a defenitive loss for the economy, which only accumulates over time. With social securities in place, a person who has recently lost his job can afford to wait for employment opportunities, during which time his consumption does not completely break off, and resume working at a later point in time.
The already alluded retaining of consumption during times of crisis is also an important benefit, and the major reason why the European states with a strong welfare state initially didn't have as much as a problem with unemployment rates during the first two years of the recent crisis (which in the meantime has of course been made obsolete by the moronic policy of extreme austerity imposed by Berlin & Co.).