A few points about Linux, Android and Task Killers:
1. Android is based on Linux, but the Dalvik byte code layer and framework design makes the OS it's own entity.
2. If all apps were designed to spec, there were no fragmentation and they "played nice", then a task killer would not be needed. Since a lof apps do not and there is fragmentation, a lot of apps do not play nice. Even OEM loaded apps can suck the resource soul out of a device. The Sony Play is an example of this (great device slowed by AWFUL bloatware).
3. Task killers are very handy to kill rogue apps that are an issue and the key is to exclude system-apps, which are usually the little green & white icons, but not always.