The primary reasons I would suggest a LNBF over a feedhorn with LNBs is simplicity and cost. A feedhorn with servo motor polarity and quality LNBs will always outperform a LNBF or an orthomode on a motorized dish. The ability to fine tune the skew to optimize for each transponder provides the best solution for the advanced hobbyist. If you are only watching the typical program distribution transponders, a LNBF is the way to go.
There is no substitute for reflector size and surface accuracy. A feedhorn will only optimize the capabilities of your system. A 7.5 foot dish is still not 2 degree compliant and this will be the greatest limiting factor when trying to receive a transponder with overlapping frequencies from adjacent satellites. No feedhorn will change this fact. Your choice of feedhorns, LNBs, filtration and other hardware will depend on the type of viewing you are interested in and your budget.
Why buy a high performance Ferrari if you need a daily driver pick-up truck? Why by a workhorse pick-up if you desire a fast Ferrari?