Not everyone is as politically aware and astute as many here. If someone wants to vote for, say, three people (s)he really wants in but really wouldn't know who else to vote for ... If they think they HAVE to use all 6 votes, they are much more likely to cast the 3 votes left by voting for someone whose name rings a bell, i.e. the sitting members. It becomes something like choosing toothpaste - unwillingly you will not go for an unknown brand name.
I, therefore, agree that - if you're not sure about your other votes, it is better not to use them ! If you are sure, by all means do !