But the caster isn't spending every round convincing the Fighter he should attack more.
Effectively, yes they are.
The caster, with haste, is "encouraging" the fighter to get an extra attack every round.
Edit: Particularly if it's a bard. I can just hear him singing "eye of the tiger" the whole time he's concentrating, until some smacks him upside the head to make him shut up.
Every spell they cast to buff an ally is one less spell they have to do anything else that might be needed. What resource is the Warlord working with?
One way is to look at what level is required to make a support spell effectively at-will (i.e. enough slots to have it running every battle). Then copy it and reduce it a bit since there's greater flexibility. (You can't switch spells easily).
For example....
A cleric 5 can have bless (+1d4 to 3 people) every combat, and 2d8 (9) sacred flame.
so..
A marshal 5 could use his bonus action to give one person +1d4 to-hit, and deal 1d8+3 (7.5) as his action.
A wizard 11 can have haste (1 attack, double speed, +2 AC, advantage on Dex) running every round, and 3d10 (16.5) firebolt all the others.
So...
A marshal 11 could use a bonus action to grant 1 attack every round, and do 2 attacks 1d8+4 (17) as his action.
A bard 17 can use foresight (advantage on attacks, and defense), plus all the other slots.
So..
A marshal 17 could give advantage to anyone who attacks the same target as him, without using his bonus action.
That can give you a starting point.
Then you playtest it.