Nancy Reagan's quick tour of drug rehabilitation programs has served two useful purposes.
* It has drawn attention to the fundamental need, as stressed by Mrs. Reagan, for parental responsibility -- including, we would add, parental example - in controlling young people's drug use. Young users can do much to reform themselves, of course, as indicated by many on Mrs. Reagan's tour. And parents need the support of institutions and other parents when drug problems arise.
It was thoughtful for Mrs. Reagan not simply to play the outsider amazed by what she found out but to associate herself with the troubled parents. She was reported to have said she is certain her own two children have used marijuana though not extensively. She promises as First Lady to continue the fight against drugs. Thus there was weight to her words when she told members of a family antidrug group that ''you're not alone.''
* The tour publicized Nancy Reagan's more serious and socially concerned side. Thus it helped offset her White House image of preoccupation with high-style fashion and elegant place settings while her husband battles the nation's economic decline. It will be encouraging to see her make further use of the First Lady's potential for symbolic and substantive leadership in addressing human need.