The simplest reason I skimmed work email on my phone was I didn't bring my work computer on vacation. But even if I had it with me— which I do on some leisure trips because they're combination work/leisure— I'd still do the skimming on my phone. Once I open my work computer it's way, way easier to get sucked into something that takes at least 45 minutes. As you note, the phone's not good for composing lengthy messages. That's a natural brake against going long. The phone's perfectly fine for triage... and I'm fine with checking once a day or so to do that.