The components are well-conceived and easy to use, and INS paid attention to extensibility. Like that famous bunny, it just kept going, and gave us the best view of our applications' performance. It is an excellent choice, especially if you want SNMP performance management as well. The combination is compelling. As with any nascent software system, vendors have found more than one way to skin an application.
Some products employ both.
Active monitors give you control over when and how data about your performance data is collected. If you flood your servers with transactions just to see where the breaking point comes, active probes can do the job nicely. However, you must rely on vendors or roll your own transactions. Further, you are limited to the protocols supported by the vendors. Some just do IP, for example, and that may not be sufficient.