For me, it is a balance between workflow and quality of final results...

In a perfect world, you could do it all in the keyboard, but there would be a nice 27" monitor attached, to make seeing everything you need easy on the eyes. And a mouse input, to navigate with quickly, and make quick selection possible.

But short of some of the TOTL 'computer in a keyboard' instruments (Neko, Mediastation, etc.), the price you pay for being able to sit at a keyboard and do it all is FAR less ease of use. And ease of use is what it appears you are after. One of the problems you have with a 'one box' solution is that, all you get is what that one box can do. If this is up to your standards, I guess you are good to go. But personally, I find that the arranger output is only the START of the process. After that comes considerable editing and replacement of arranger parts with played ones, addition of considerable loop audio, and then replacement of the arranger's sounds with better sounds from either other keyboards, or VSTi's.

THEN... you have whatever audio you want to record, from as basic a thing as a simple vocal (never really that simple if you want to comp together multiple takes!) to backing vocals, to real guitar parts. And here, the computer is king. No arguments at all. From easy comping, to insert effects far beyond what the arranger can do, to ease of identification of what track does what, to a track count no arranger can match...

In fact, there really isn't an arranger that can handle multi-track audio without horribly convoluted workarounds. Most of them have simple 'one take' audio recorders that offer little control, if any. If all you want to do is record your voice singing the song (from beginning to end, better not make a flub in the middle!) you are probably good to go. And if that's all you intend to do, sure, have at it. But I think you'll find yourself wanting to put on a bit more polish than that, after a while.

Maybe you just want to record quick and dirty song demos, roughs that won't see the light of day, but if you want to record something more professional, I'm afraid a computer is going to have to be a part of your arsenal. Look, it might seem a bit daunting to start with, but it's just the same as mastering any modern TOTL arranger. It's going to take a while. But, at least with the computer, there is no artificial upper limit to what you can do. You'll be banging the ceiling on any modern arranger, trying to record a complete song on it.

And, in the end, the computer you buy will be a tiny fraction of what something like a PA3x or T5 is going to cost you, and you can always go laptop, if you can't get access to the house computer for music making. But it's power is exponentially better.

In the end, it comes down to what you want to do. If all you want is a quick sketchpad, the arranger will do in a pinch. But if your plans include any audio and a finished master, you are going to find arrangers severely limiting.