Many devices use more than 0.5a while in use. To charge a device like that on the 0.5a port you have to turn the device OFF while plugged in.
For example say phone A uses these values (made up for the example)
off = 0a
sleep = 0.2a
screen off device awake = 0.4a
screen on device awake = 0.8a
screen on CPUs fully busy = 1.2a
the rate of charge on a 0.5a port would be
0.5a
0.3a
0.1a (as in your charge rate would be so slow you'd never notice)
-0.3a (as in your charge would drop over time instead of increase)
-0.7a
If you don't have an adapter to plug into the cigarette lighter power port and really need to charge a phablet, tablet, or iphone then turn it OFF so the 0.5a is enough to do some good.
Then when you decide that isn't enough buy an adapter from amazon and enjoy.
personally I went with http://www.amazon.com/gp/product/B005LFXBLE?psc=1&redirect=true&ref_=oh_aui_search_detailpage back in 2013 specifically because it offered more than 0.5a and less than 1a. I don't want to charge my batteries in my phones too fast (slower charge equals less heat/degradation). There are times when my phone is busy doing GPS/cell stuff on a trip that it's using more than my adapter is putting out, other times it is gaining charge.
I'm happy enough with the charge rate it has even with my newer phones but if it ever isn't enough I'd get something like this http://www.amazon.com/Charger-Coating-Sentey-Ls-2241-Powerful/dp/B00S00D7U2/ref=sr_1_6?s=wireless&ie=UTF8&qid=1440816164&sr=1-6 that has both 1a ports and a 2.4a port so I can choose to charge slower or faster. Odds are only one of my devices is really low on charge and even in the worst case I could plug both in and swap power low/high between two devices periodically to balance the charging opportunity as needed.
The point is no matter how much power the port in the car gives there isn't a single right answer for how much power I want for all my devices. I want to give some of my devices less than 1 amp. I may buy a new larger phone in a few months that I'll want to give more than 1 amp. But even then I'll still have more than one device and will need to use multiple ports to have a choice on how fast they charge vs what I need.