Are there any reasons not to use a 10w ipad charger with a USB->USB-C cable to charge a new MacBook Pro 2016? Have the one charger that came with MacBook but would like to be able to plug in at office, home on road, etc. with other chargers. Thx.
A 10W iPad charger should in theory work and you would be correct that it would be a slow charge. Well, to be accurate, it would be a horribly slow charge - you are getting a charger that is 1/6th (17%) of what is required (61W).
Will there be a problem? No.
Will it charge? Slowly. Molasses in January slowly. From Apple's web site:
Your Mac will charge from USB-C power adapters not manufactured by Apple if they adhere to the USB Power Delivery specification.
Assuming you are using a genuine Apple iPad charger, it should charge since Apple charges adhere to the spec.
A 10 watt iPad charger will not charge a MacBook Pro because the MacBook Pro will only charge from a USB power supply that supports "USB Power Delivery" or USB-PD. Apple's website states as such. Even then there may be other conditions that need to be met, such as voltage and current supplied.
No USB power brick under 18 watts, at least none that I am aware of, supports USB-PD. A USB power brick that doesn't support USB-PD might supply power, which will slow the discharge from the battery, but it will not add charge to the battery. Connecting a 12 watt USB power brick to my MacBook Pro will show in System Information as supplying power (at least that's what it looks like to me) but it does not show as charging the battery.
During a power outage I was able to run my MacBook Pro from a 27 watt USB-C charger that was plugged into a portable power pack. With some rather lightweight web surfing I didn't see the MacBook battery run down. This same 27 watt charger was able to charge the battery while the laptop (and I) slept.
Officially that's the story, that a MacBook will only charge from a USB-PD power supply. Unofficially, I have read that people have had mixed results using non-USB-PD chargers to charge MacBooks. Personally I have not been curious or desperate enough to run this experiment myself. My guess is that for this to work the laptop would have to be using less power than the power supply so that there is enough power left over to put in the battery. That means the laptop would likely have to be powered off or asleep.
If you are looking for a travel charger for your MacBook Pro then I'd suggest getting nothing smaller than 27 watts. Anything smaller is unlikely to charge the battery, and if it does it will charge quite slowly.