Cover 2 is bugged and can't be taken. Melee is fine and we don't want 100% mounted or more rock paper scissors. Melee are good meatshields and units that can stand in front. Defensive bonuses and cover can make them very tough. Some melee as anchor and mounted as mobile support with ranged support is quite good. There is another thread saying mounted is useless so its hardly universal.
Not in my game. Here's an easy fix.
Go to the following directory (whichever drive it is on):
Steam\steamapps\common\sid meier's civilization v\Assets\DLC\Expansion\Gameplay\XML\Units\
Find the file "CIV5UnitPromotions.xml" and open it with Notepad.
Scroll down til you find the section <UnitPromotions_UnitClasses> and add the following lines BEFORE the first <Row> (make sure you add it between <UnitPromotions_UnitClasses> and the first <Row> otherwise it may cause crashes if you got a misplaced <> begin command without the </> end one.
<Row> <!-- New -->
<PromotionType>PROMOTION_COVER_2</PromotionType>
<UnitCombatType>UNITCOMBAT_MELEE</UnitCombatType>
</Row>
<Row> <!-- New -->
<PromotionType>PROMOTION_COVER_2</PromotionType>
<UnitCombatType>UNITCOMBAT_GUN</UnitCombatType>
</Row>
<Row> <!-- New -->
<PromotionType>PROMOTION_COVER_2</PromotionType>
<UnitCombatType>UNITCOMBAT_ARCHER</UnitCombatType>
</Row>
<Row> <!-- New -->
<PromotionType>PROMOTION_COVER_2</PromotionType>
<UnitCombatType>UNITCOMBAT_SIEGE</UnitCombatType>
</Row>
That should fix it... the <!-- New --> is just a comment so I can quickly glance through the xml to see if a patch overwrote it. You can indent the lines to match spacing with the existing ones but it isn't absolutely necessary. The bug is they left out the classes capable of chosing the Promo so it can't be chosen at all (no classes are eligible). All I did was copy the info they had for COVER_1 and repaste it changing 1 to 2.