My jump infantry usage depends upon whether I'm using canon 21-man platoons or not. There's nothing in the rules that prevent jump infantry from having 28 men like other canon platoon types, and as per the construction rules in TM they can go up to the maximum 30 per platoon.
With larger platoon sizes you don't suffer the loss in damage capacity and firepower imposed by the 21-man organization, thus allowing you to use jump infantry as your general purpose infantry type for all bar those troops that you want to equip with field artillery/guns. The one exception would be for troops intended to operate in terrain where even jump infantry will have problems, such as large bodies of water, where you'd be better served using mechanized hover/VTOL infantry, appropriate beast-mounted infantry or scuba infantry. The one caveat to this is to make sure your APCs have the capacity to carry the larger platoons.
With the smaller, canon platoon sizes then jump infantry aren't always going to be the best choice where raw firepower is desired. They have no limits on the infantry weapons that they can wield per the rules - with the exception of the Support PPC, which kind of sucks anyway, so that's no loss - but you're always going to have just three quarters of the firepower of foot infantry, assuming the standard 21- and 28-man organizations. Likewise, your ability to absorb damage will be similarly weaker; weapons which might wipe a jump infantry platoon can fall short against foot infantry, giving you another turn of use of that unit.
In such cases, jump infantry are still good in circumstances where firepower isn't as vital or where their mobility provides adequate compensation. Using them as spotters, whether just simple observers or when equipped with TAG, is a good example. Another possibility is Inferno-armed anti-battlesuit troops, but this depends heavily upon your primary weapon if you're aiming for one-shot kills against enemy battle armor squads. The weapon of choice is the Mauser, whether the 1200 or (preferably) IIC version, giving decent to excellent range, which combined with one 2-shot SRM launcher per squad will provide a 21-man platoon with enough firepower that they can achieve 12 Inferno hits per salvo when at full (or near full) strength. This is enough to auto-kill any IS squad of battle armor, with the exception of those equipped with fire resistant armor, of course. Such platoons have the mobility to match or exceed most jumping battlesuits, making them a brutal option against suits like the Ravager.
