What is the fixation with "some ON, the rest OFF"?
1) PWM turns ON *and* OFF 490 times a second.
2) 10 leds all at 10% PWM uses the same power as 1 led ON all the time.
3) 10 leds all on 1 PWM actually gets you 10 leds with 256 different light/power levels.
4) 10 leds set up so that you can choose how many light up gets you 10 different light/power levels.
5) 10 leds set up with PWM each gets more levels and maybe with enough leds you can see a difference.
6) Many leds on 1 PWM and transistor is simple wiring and simple coding.
7) Many leds with individual control uses many more parts, more code, and is suited for fancy displays.
You can use "software PWM" to increase the number of levels past 256 a few times over if you want.
Are you trying to save power? Usually that is to save money. If so then why spend a bunch more on the lamp?
You could run your lamp with an 8 pin ATtiny25 and a few more on the same less than $1 chip.
What is it you really want to do?