Unhelpfully: the Beginners Box game can be played just fine without record sheets
Only if you play using stripped-down rules which are useful for introducing game concepts to new players but not particularly fun as a game. (Recently got my brother to try BT, he did say after intro game that next time with full introductory rules.)
Or if you go full calvin-ball.
For BT proper, you need record sheets. Canon sheets are the simplest and most convenient source for units and playing the game. The suggested method. RATs, scenarios refer to official sheets.
Official units are easy to use, they're mostly OK, not too good and not too bad few outliers aside. A casual player might read a TRO, note a described variant and decide they'll use that. And they'll be disappointed to find there is no RS for that unit. I very much doubt they'll start making a custom unit from a description. Not to mention a casual player probably isn't adept at making units that are reasonable whether or not they have a vague description of a unit.
And even if these are not issues, how many are going to make custom units before a game? Much faster to use canon sheets.
Which really sucks when you want to play a Dark Age scenario with smattering of latest tech and can't because the stats are not available.
EDIT Furthermore, no record sheets for units in official TRO is a bad precedent.