I think the idea that "pure" casters on average have lower physical skills is a holdover from other games that force classes to have lower hit points and have otherwise arbitrary class-based restrictions.
HARP doesn't
force class stereotypes like D&D does, but there are still solid game-mechanic factors that encourage particular character types into particular development paths.
And, IMO, one of the best things about HARP is that it is just a fairly gentle push rather than a rigidly-enforced rule. That's also what I liked about RM when I played it in the 80s and 90s.
Spell-casters in HARP
tend to have lower Combat, Athletic, etc skills
because they're non-favoured categories and cost double. Also, they have already have lots of skills (i.e. spells) to spend their points on. In HARP, "opportunity costs" are significant - every DP you spend on one thing is a DP you can't spend on something else.
Same with Endurance - every DP you spend on hit points is a DP you're not spending on a spell. Or PP dev. Or Runes/Alchemy/Spell Lore/etc. And once you get to 10 ranks, the incentive to buy more ranks drops from +5 to +2. So, 5 or 10 ranks plus racial and stat bonuses makes sense for many spell-casters....and where a fighter type might drop any "spare" DPs they don't have an immediate use for into Endurance or a combat skill, a spell-caster usually gains more benefit from dropping them into PP dev or a spell.
As for Armor, there are obvious reasons why HARP spell-casters wear little or no armor, and invest very few points in the Armor skill.
Of course, casters could do the sensible thing and buy the
Additional Profession talent to take a level or more of Rogue or whatever to concentrate on their Athletic, Combat, Outdoor, or Subterfuge skills (with the opportunity cost of a level or more of
not improving their professional spells). It's 20 DP for that talent (or 15 if you're a human with
Professional Adaptability), but it pays for itself if you intend to take more than 10 ranks of non-favoured skills (less, actually, because Add. Prof. also gives you your choice of one professional talent, bonus, or sphere. e.g. taking a level of Fighter is a pretty good way for a Mage to get
Lightning Reflexes or
Shield Training, both of which cost 10 DP,
AND rapidly improve their Athletic and Combat skills).
IMO, every caster should have
at least 5 or 10 ranks in at least one weapon - even if they only ever use their quarterstaff or dagger for parrying in an emergency. A good, solid whack with a
qs is often more effective than a fire bolt or whatever, anyway. And having a bow or something as a secondary weapon gives them something useful to do in combat if they run out of PP.
Additional Profession is a fast way of getting those ranks, otherwise buy them slowly at 4 DP per rank over a few levels.