"If all you have is a hammer, everything looks like a nail" [para]
The idea that a developer can be an expert with only one tool would be like expecting a carpenter to be an expert with only a saw or chisel and the world of finely crafted furniture would be much the poorer for it.
And to mix metaphors just for a moment, the notion that a person can drive almost any vehicle in a car park would seem a claim not worth making - you'd take it for granted. Yes, each manufacturer may place the light switch, indicators and wipers in a different place and the gear configuration might be slightly different, but they are all things you know will be there, you just have to spend a few moments to find where they are.
If what you need is something to work fast, efficiently and effectively and has the scope to develop, then you'd better choose someone who knows which tools are best for what and how they best work in combination. A screwdriver expert is only ever going to be able to assemble flat packs.