Okay, very shortly after I posted this question, I figured it out. One needs to simply remove the 'marker' option from the command. So, even if you want to access a vector of symbols, you can just state that directly. For instance:
plot(1,1,mark_sym(i)); legend('Case 1');
In this case it will find the symbol in the vector mark_sym and use the appropriate symbol. One does not have to say:
plot(1,1,'marker',mark_sym(i)); legend('Case 1');
which will create an extra line through that marker in the legend.
