Hello Eitaknella, I believe I will be able to help answer your question and concern! It isn't necessarily good for your battery as it puts a strain on from trying to charge and draining at the same time. However, I tend to do the same thing. I have had my iPhone 6S for almost a month or so now and I haven't seen a noticeable difference in my battery, as far as the life of charge nor the rate at which it charges. Although the effects may not be noticabl immediatley, it will put extra wear and tear on the battery.
It can also cause the battery to rise to unhealthy temperature, depending on how demanding the phone use it to the processor and graphics card, which can possibly rupture the internal cells of the Li-On battery which can cause it to be greatly damage and perhaps ruined. So to make a explanation brief, I wouldn't recommend it, but also see it as a compromise to consider. I believe the inconvenience of not using a phone while charging is a large hurdle to overcome, especially since I am always using my phone throughoit everyday.
Hopefully I have helped answer the questions you had in depth and helped you conclude a decision on how to handle and care for the life of your iPhone internal Li-On battery!
Here is a link to an article on the topic you have questions about in addition to my personal opinion as well the information I have provided. The knowledge I have gained has been acquired through being a 94E(Radio ComSec Repair Technician) in the U.S. Army as well as a certified Direct Current Electrician, with approximately 5 years of experiance in that trade.
With Concern and Sincerity;
Landon P Johnson
US Army Veteran