I'm not sure that having higher tiers should necessarily result in better chances for those assignments. Effectively, access to higher tiers gives us access to better DOFF's and that's what should affect them. I'd like to see more DOFFs in the DOFF store; hopefully we'll get them down the road.
Besides expanding on Transwarp destinations, other benefits I can see are things like:
* Passive Bonuses (like, Upper tiers in Trade giving Ferengi-like bonuses in dealing with vendors. Or high rank in Espionage giving bonuses to stealth)
* Costume unlocks
* Ability to replicate special consumables
* Ability to replicate new holodoffs
* "Cheaters" like the Diplomatic Immunity power that lets you do something you normally can't do
Expanding on that last point... It would be mighty cool if getting high ranks in Military, Engineering, and Science let you complete optional Mission objectives as if you belonged to that class. Or let you use kits you normally can't use. Or granted you a class-specific power you normally don't get.
Obviously they can't sweeten the pot too much, or DOFFing becomes OP and Must-Do.