Holding tower is very risky business. Unless you're a CC heavy or self defensive support support (Leona, Janna, etc.), holding tower is very risky business. The duo can easily take advantage of you being alone and in most cases, will dive you. And if their jungler is following up, it's almost certain they'll get a kill, possibly without even sacrificing one of their own. You're better off backing there or roaming around, taking the chance to ward, and perhaps assist your mid lane in achieving a kill, or perhaps even pushing if the enemy laner backed.