The developers responsible for the device and its software also cannot control what develops as far as apps. Apps with memory leaks, apps that use sensors, network, processor time, etc, in the background, and apps that don't close themselves (most) all add to the formula.
As I've stated, and I'll repeat, if you're downloading a lot of apps like those I've mentioned, and not paying attention to the settings, and/or downloading known background resource hog apps, having a task killer is a good idea.
It's easy to create a repeatable experiment, where I open 5 apps on my phone, push 'Back' out of each one, and then hand you my phone.
After 15 seconds of you not being able to stand how slow it is, you can 'kill all', and watch the difference.
***It's laughable to say that "Android 1.5 doesn't need a task killer", given the plethora of poorly written/resource hogging apps that are easily and openly available on Market. It's not a "Needs or doesn't need" issue. Not everyone will benefit a task killer. Again, it depends completely on the apps you use, and how you use them. Configure your apps conservatively, and avoid known resource hogging/sensor using/partial awake programs, like many users on this forum who are saying you don't need a task killer, and your phone will work just fine. Without a task manager, MY phone CAN be laughably laggy. I use a lot of apps (including Remote Desktop and VNC apps for work), and manage which ones are opened. I have to. Many people don't.