The recommended supply is in fact the maximum amount of current that the Pi 3 can use; if you try and draw more than that you risk tripping the main polyfuse. On other models this is actually 2 A.
I think the reason they (the Foundation, distributors) make recommendations like this is because if they say stuff like "Minimally _____, and no point in more than _____" people who go with "minimally" are more likely to end up buying something that doesn't work than if they just say "ideally", partially because not all supplies are created equally. If you buy a decent quality, 2.5 A, 5 V supply intended for powering electronics (vs. just recharging batteries), you are likely to be happy. Hence the recommendation.
But no, you do not actually need 2 A to turn on a Pi 3. You could turn it on with 1 A, in fact, although a 1 A charger may disappoint.
In my experience chargers usually work with Pi's, and I have (several) ~2 A ones which work with a Pi 3 (note: most of them are 5.25+ V), although I mostly use 2.5 A 5V supplies (not chargers) and have not bothered to do serious stress tests with the chargers. I am contrasting this to 5V 2A power banks, which seem to do okay with other models but have failed for me with the Pi 3. I've speculated about why that is and much of that logic would apply to chargers, except for the fact that they aren't stepping up lower voltage batteries.
Simply booting is a reasonable 20 second stress test. Here's what I can tell you using a charger stamped 5.35 V, 2000 mA, based on a cheap USB ammeter:
- Reports 5.15 V with no load.
- Voltage drops to 5.09 V with Pi 3 attached.
- Peak consumption during boot is ~450 mA.
- Consumption at idle is ~250 mA, headless, no peripherals, wifi on.
The last two numbers I think concur with other reports you will find online.
To summarize, chargers are explicitly intended for charging and YMMV using them as supplies. They probably can provide up to their rated amperage, but you may encounter the wrong kind of curve with regard to instantaneous delivery as you approach that limit. Unlike active supplies, there's no reason for them to be responsive in that context.
If you are going to buy something, buy a supply. If you already have a charger and you want to see if it will work, try it. The red light should remain steady and strong; if you are using a display there should be no little rainbow square or lightening bolt in the upper right corner (this is a low voltage indicator, which will happen if a supply cannot deliver instantaneous current effectively).