There are actually 2 modes which make use of this thresh value, VARIANCE_EXPLAINED and THRESHOLD. So we can't do away with the EPCAMode input in this default constructor. So a user has to specify Mode as well as thresh value.
This constructor also comes in handy when user wants to set nothing and go with the AUTO method in FIXED_NUMBER mode (which is most likely). All he has to do is CPCA() or CPCA(true) [if he wants to whiten results as well].
The next most likely thing that a user would do is change between SVD/EVD. For that he has to do just CPCA(SVD)/CPCA(SVD,true) [the other constructor is for these calls].