Given the use of the word 'when' I assume you mean in campaign. I'm pretty sure you've played more Germania in campaign than i have, but my fast answer would be ... never - unless you were deciding to go all cavalry.
In the campaign, you are typically better served by durable units that can tie down and chew up masses of enemies than high attack-drop like flies units ... like chosen axemen. They win well enough, but how many of them are left? Base axemen I consider pretty much a waste as germania. Chosen axemen have definite uses, but I imagine I'd always be using them as a complement to spear warbands. In field battles, I do like some second-row chosen axemen to plug holes - I use them to counterrush when opponents units break through the warbands. In campaign, those units that break through the warbands tend to be heavy units -ones that are deserving of chosen axes' ap attack.
In city battles, Chosen axemen I lob at walls, spear warbands in the streets.
Considering other german infantry ... berserkers are, as you've noted, of limited use due to their unit size. Night raiders are decent, but are better used as a fast scary unit (to me) than as a backbone - they just aren't as solid as the spear warbands as an 'anvil'. In any case, the spear warband/chosen axemen 'retrainability' when taking foreign cities is a plus. I prefer to focus my 'custom structure' building, when attacking, on the temples needed for gothic cavalry - which rather then being a 'specialist' unit are a welcome upgrade over barb cavalry.
Chosen archers as a backbone might be a more interesting choice - but against whom? Against rome, I'd rather have a mediocre phalanx than an archer I think, romans have too much armor. Perhaps against other barbs, but then again, most barb units are such fodder to spear warbands there's little incentive to switch.
While night raiders, berserkers and screechers are all cool specialized unit, to me its the spear warband as a backbone that makes germany far stronger than say ... well Gaul ... in campaign. In my lone german campaign I took all of italy using spearwarbands/chosenaxes/gothics.
I'm interested to read more of your thoughts on it though, as the resident germania-phile.