Its mentioned as not being codex approved, as in the Codex frowns on units being so specialised.Lord Revan wrote:aren't the Tyranid war veteran squads another formation that's strickly speaking not codex approved, yet it's something that the Ultramarines use (or at least used during previous editions) and that doesn't have the excuse of being a pre-heresy tradition.
Otherwise, once the Ultramarines had recovered their strength, its hard to see if any controversy exists org chart wise.
You probably should refer to White Rabbit or the other Wh40k veterans, but the fluff isn't consistent on this.As far as I remember the armour elements where separate from infantry elements of the chapter. Taking troops from the reserve companies for armour detail takes away from the 106 in each company the codex calls for.
It's true members from the 8th company do serve in land speeders that seems to be the exception in terms of armoured units rather than the rule if I remember correctly.
In theory, the 1st Company is the only company to have Land Raiders permamently assigned to it and heavy armour are drawn from the Chapter Armoury. Rhinos are organic to the company, and as you stated, bikes and land speeders are assigned to the Assault Company permamently as part of their training.
So, those marines that man the tanks have to come from somewhere, its either the company they're attached to or they're drawn from the reserve companies.
Basing off Lexicanium, it seems that the Spear is unique in that he commands 50 other Marines directly, as a permament tank force.