It creates 5 tasks and what ? It changes nothing really.
The code for set_task works like that : AMXX hooks ServerFrame, does a check so the set_task code is executed each 0.1 second, then this code loops on all registered tasks.
5, 10, 15 tasks, you won't see any significant changes. All checks and such are done in the module, it's optimized and fast.
Your way to use a global think, could be useful only if you need to use a think for 0.1 second _only_ in several plugins, so you would save entities. That's only the advantage I can see.