Altcoins are effectively the 'marketplace of ideas' where new concepts and challenges of launching, sustaining and managing a cryptocurrency are being experimented with. In the early phases, there were very few ideas about how a Bitcoin-alike should function -- now you have many different variants, with hopefully more to come that provide true differentiation.
Are there too many 'junk coins'? Sure, because so many are nothing more than 'another coin with a different name' or a venal attempt to cash in on the crypto-craze with a quickie pump-and-dump. But many of them bring innovation and test different theories of virtual currency economics. Look at the success of DOGE as an example -- it started effectively as a 'joke' but is continuing as a 'serious' crypto. Directly copying that, however, as CAT and KITTEH have attempted to do isn't going repeat DOGE's charm or success (sorry cat-fanciers!) because the community supporting it sprung up organically via Reddit and supported the coin without a concerted effort to do so. But analysing why it has succeeded thus far is a good way to consider how to launch other cryptos, or how to improve current ones.
Or consider HunterCoin, another interesting 'proof of concept' which brings something entirely new to the table -- a coin with a built-in game and economic engine.
the tl;dr: some altcoins may be bad; but many are useful, even if not successful.