First and foremost, go to the key setup and unbind the “Select Army” hotkey completely. Yes, you already know about the control groups, but old habits die hard and you’ll still be using F2 by accident here and there, unless you unbind it.
Well, you should better use the first numbers for armies and the other ones for macro, not the other way around. arms has a point about the “1”, “2”, and “3” buttons being closer to your hand most of the time — and, believe me, when you’ll need to fight here and now, you’d rather control your army rather than your base 
I play as protoss and here’s how I use them:
“1” — the whole army
“2” — specific casters with this army (High Templar or Disruptors)
“3” — a scouting probe (early game) / warp prisms (mid and late game)
“4” — Nexi (for chrono boosting and constant probe production)
“5” — some unit production buildings (Robo or Stargates, depends on the build)
“6” — Forges or CyberCores for upgrades
Well yes, but actually no.
You may leave some army units in behind for defense (in particular, to fight off the liberators or oracles). You also may use some units for scouting or holding Xel’naga towers. Not to mention that you may attack on multiple fronts. All this will be possible if you use the control group instead of F2, since F2 will just move away all those stategically positioned units.
Yes, the control group has a downside of your freshly made units not coming along with the rest of your army right away, unless you add them manually. But hey, here’s an upside: when your army is far away behind the enemy lines, or you’re just losing and have to retreat, you can rest assured that at least your new units will be safe at home. And besides, you can always use them rally points to order them to move out approximately where you need them.