Category ranks + static profession bonuses ends up serving the same function as progressive level bonueses in RM2.
While an RM2 fighter gets +3 level in weapon skills for life, a RMSS fighter starts with +20 then purchases ranks in "weapon category X" to get the rest.
It does allow for a bit more customization than fixed bonuses, since you can choose where to spend your category ranks, but in the end 90% of spend is done within archetype, so the end result is the same, with a bit more paperwork.
Offering an example:
1) Take RMC using the "RMFRP friendly skill categories" option.
2) Give all players +30 DP per level.
3) Characters start at 1st level with fixed 5th level level bonuses. So +1 = +5 bonus, +2 = +10 bonus, +3 = +15 bonus and +3 open ended (Fighter "Combat" bonus) = +20 bonus.
4) Players must purchase all further level bonuses each level at 3 DP per +1.
5) Players must buy +10 in bonus each level.
There would be a slight variation in the bonus per ranks level compared to RMSS, and how diminishing returns affects that level bonus, but that seems to be essentially the concept behind RMSS category bonuses in a nutshell, from my point of view.
It's neither a good nor a bad thing, I'm just not sure that it's the best way to achieve the end result desired. (It's certainly infinitely better than "Similar skills".)