'leave them running indefinitely' isn't really clear.
When you leave an Ubuntu machine is goes to sleep and uses barely no electricity. For me, I just shut the lid when finishing and the laptop suspends to disk - then when starting work I just open the lid, enter password and carry on from where I left off.
You're probably only used to Windows poor way of trying to do the same - for modern OS's things are much better - after all the guys who can write such a brilliant OS can also sort out sleeping and suspending properly.
This is why the boot times aren't that important - I hardly ever reboot. And I don't think my wife has rebooted her Mac in months!