Depends on how many pockets and locks you want to pick, especially at low level. There are only so many skill points per level, and most go into HiS/MS. Having only one thief forces you to pick and choose whether you want to be useless at picking pockets or opening locks, especially since you also have to sink points into finding traps (although, in practice, finding most traps takes more time than actual skill; if you wait long enough in the area, you should find 80-90% of them with even 30% skill; the others, of course, will kill you dead.) Nalia is a good example; she picks pockets OK, but sucks at everything else, even stealth.