I have to agree with some of what has been said already. A basic 4-channel radio is fine for trainers and simple planes, but can quickly be outgrown as you advance.
If you're starting out with one of the 'RTF' .40-sized ARFs which include a basic 4-channel TX, so be it. Use that radio. There's no point in spending a bunch of $$ upgrading the radio - at the beginner skill level it won't make much of a difference.
OTOH if you're picking up your components seperately there's no point in going barebones on the radio. A low-end computer rig doesn't cost a whole lot more than a 'midrange' analog 6-channel radio, and will generally have features useful not only with your trainer but potentially with "your next plane."
Another choice is to buy two four channel radios; one with flight pack and the other just the radio only. Make sure the two radios are on separate channels than the other; for instance make one a 58 and the other a 54.
I have to
disagree with this idea. Why start out with barebones radios on multiple channels? Spend the $$ for a decent main TX. If you want your own buddy box keep an eye on the swapshop. Compatible barebones buddyboxen turn up there on a regular basis for $40 bucks or less - why drop $70 or more on a limited-use spare?