The apps I mentioned are in no way "essential processes/apps that are part of the OS" and they do use battery power and wireless data even when I need to save them.
You mentioned the camera app - which doesn't run in the background, but the camera frameworks may still be resident (but frozen) in memory if it has been used recently and those are part of the OS.
I can receive emails, VoIP calls and texts, updates from my city's transit service app
All of those are handled by the system multitasking processes which are part of the OS. POP emails are only fetched when you open the email app (or on the interval you've set in Settings), but the app itself may still reside (frozen) in memory if it has been used recently. Background processes exist to receive Push email notifications. As the purpose of a Push email account is to receive notification of new emails instantly, something has to be running to receive those notifications. If you don't want instant notification of emails, don't use an email account that is designed for that purpose, or turn off notifications in Settings.
Since incoming VOIP calls are initiated by a third-party, something needs to be running to receive them as you won't usually know when you'll be called. VOIP apps can be disconnected (signed out of) from within the app, thereby stopping the processes that run to receive VOIP calls.
If you start a new app, and it requires more memory to run than is currently free, the OS will automatically close the least recently used app(s) until enough free memory is available. It's not something a user needs to do.
Every phone on the planet capable of receiving text messages has to have some process running in memory to receive them, otherwise you wouldn't get any texts. As receiving texts is a fundamental function of any mobile phone, I don't see why you would need to stop that process.
Updates (push notifications) from transit and other apps can be configured and turned off in the Notifications section of Settings.
Requiring a task manager is not the Apple-way of doing multi-tasking. See: http://www.engadget.com/2010/04/08/jobs-if-you-see-a-stylus-or-a-task-manager-th ey-blew-it/
If you don't like the way Apple designed multi-tasking in iOS, other phone operating systems may be more suitable for your needs.