No, I haven't used a "drag" for all of ours. BUT the ones that I have, have been easier and better later when I went to carts. It's also one extra step to help with getting mine more reliable or used to more "stuff" before taking off in a wheeled vehicle. Also - most of mine are quite a bit larger than your minis - I like that extra step to make them solid and to see where they are before I "ride behind them"...
For me, several things - a drag was easier to do than a set of shafts first; getting them used to a tire also got them used to "other stuff" - so that they could/would pull other things (such as the trees - which both pull harder and are heavier than a wheeled cart). My ponies all do seem to be a little surprised when we get out on the road with a wheeled vechicle.
All that said - the filly that went with me 3 years ago when I was getting actual lessons in driving singles & pairs and then hands on help & directions with my two ponies. She had some concentrated work from December thru Feb (10 weeks) that included ground work, tarp work, ground driving and pulling some different drags. She was hooked to a cart a handful of times -in the 60' round pen. Took her with me to a plow day today - hadn't planned on hooking her as she hasn't been worked "as a pair" and I didn't have a single pleasure harness or a cart with me. BUT I hitched her next to her dam and ground drove them. It went well - and with a header steadying them as a pair, I hitched them. I then drove off with her - and had a GREAT drive w/ my little wagon for about 30 minutes.
I don't have any pics from the side or front that show our wagon - but here's a pic I took while handling the lines in one hand, right before I finished our drive. GG is the darker pinto on the right - Bell, on the left, is her dam. GG is now taller than Bell...
For me - having my ponies work with a "drag" is another step in training for them, making them more steady & solid AND makes me more comfortable and confident.
****to noises behind, maybe having a plastic sled pulled behind them, tension on the traces, and a PVC travois to get them used to shafts and something behind them. ****
and this, to me, is a "drag". They are all intended to teach them how to deal w/ pulling/pressure on the breast collar or neck collar & hames, with having something move behind them and with pressure against their sides... I just use different pieces of equipment to attain the same thing,
- including some unconventional ones.