I consider myself as a developer (actually it is half of my daily-job). And I tried to make cyanogenMOD works for FP2, but I didn’t succeed until Lineage arrived 
I am speaking for myself and only for myself, I don’t want to pretend to know the needs of other dev but I don’t like the idea of a “free” phone for dev, for different reasons.
First, who is going to judge which one needs a phone or not? As we can see on this thread, some person don’t have the “dev” badge (hi @Roboe) when they are actually doing incredible dev work, other person don’t consider themselves as “dev” while actually working hard to debug, test, etc… And imho, the part where you need a phone is not while developing, but while testing and debugging.
Developing is time consuming. But the real issue is “compiling” or “cooking” a ROM, basically waiting 4h to get a new build after you did your modifications. Flashing the ROM needs like 5 minutes and if it crashed, you can put your old backup (the one you daily-use and that is working).
Installing the requirement on Linux, download all Android files to be able to cook, etc, this is very long. Flashing is actually the quick and fun part for me, and for this my phone is enough.
It also could be seen as a kind of “reward”, and I am definitely not doing this for this. It can also be a bit stressful and lead to a kind of “obligation of results”, or at least people will want justifications (report). I can imagine someone finishing a build and don’t try it during few weeks because he (she) has other thing to do. If he (she) has a “free” FP2, I think people will ask him (her) to test, because “we give you a phone for this”… Maybe I am wrong, but I am sure it can be like this… I don’t think it is a good way to encourage FOOS (Free and open-source software) for FP2 or speed in development.
What I would prefer is for example more collaboration with Fairphone dev team (even if it is really complex to do) and most important, access to a cluster (or a big computer) to compile ROM quickly. Once again, this is the bottleneck imho.
Finally, most of people developing for FP2 have a FP2. There is exceptions, I agree, and for those exceptions we can maybe do something, but for most dev, I don’t think it is needed or productive. For testing, yes, for developing, no.
I stop developing for FP2 for almost a year, because I don’t find time right now. If I had a free phone, I would feel obliged to “work” for the community and it is not how I want to spend my free time
what problems occur they have to tackle (e.g. hardwarewise).
) , mal (don’t know his forum username) with SailfishOS and more ubports/Ubuntu Touch people but they get financed by ubports directly afaik