Bard's ult really gives you a lot of control over situations. It has so many uses that its hard to explain.
You can stop flanks by ulting the flanker.
You can save your carry if he miss positions by ulting everyone including him with the hopes that he can reposition after the duration or flash away or be saved by teammates CC.
You can catch people and chain CC them after the ult comes up.
You can bait with yourself and ulti yourself (did this in one of the wins I had yesturday).
You can split enemy teams in half during team fights
You can use it to zone enemies into walking a certain way to avoid the ult (hopefully back into your team or get zhonya'd being there options.
You can ult a ganker and stop him from kill you and your ADC
You can ult towers/baron/dragon to give your team a little more time to get to the objective to contest
The situations go on and on. The problem that his ult has is that you can really turn good situations in to horrible ones. Lets say your vlad just ulted there enemy team and you ulted on top of it.... There goes all of vlads damage. This can happen with just about every AOE ultimate in the game and its aweful when it goes bad.
Other bad times to ult are on low health enemies. You just gave him the chance to escape via flash or movement skill. Good job bro. Prepare to be flamed for 2-3 minutes by your teammates.
Every bad Bard ult is rediculed by your teammates and every good one is pretty much ignored since you are a support and supports never get a pat on the back 