It seems to me that quarkcoin is an experimental protocol. Then again, bitcoin is often looked at as an experimental protocol, and despite its performance, is still considered beta (if you are reading this and questioning that statement, go look at bitcoin-0.8.3, for example, on sourceforge, and look at bitcoin-0.8.6 (same place, most current version), as you can see bitcoin itself is plainly characterized as "beta."
Even if nobody was mining it, a decentralized protocol that is utilized for transactions would still be interesting (if for no other reason than that it would be an example of how anyone can take part in the decentralization process). It's my sense that part of the incentive to mine quarkcoin is the anticipation that it may gain in value and thus be able to be both used more widely for purchasing, as well as its current presence on certain exchanges. Another part of the incentive could be the feeling or sense that comes from having accomplished mining something.
This is not a recommendation of any particular cryptocurrency / protocol, just an observation - Suggest looking at main.cpp Quark for observations of difficulty settings. Also, the, SQL for the blockchain up until Dec. 3, 2013 is here (not my site, but contains useful information): entropyextropy.com -- at entropyextropy.com/abe.sql.zip
(Information in above paragraph taken from twitter and the forum at quark.freeforums.net)
On github, see code associated with MaxGuevara titled quark
Touching on the question of how is the security of quarkcoin increased by adding hashing functions, I'm not certain that this is how security actually works. I'm of the mind that the higher the adoption rate of any particular currency / protocol type, the more it will be able to weather attacks of many varieties.
[Note: See topic=260031.1840 at bitcointalk.org]