There was at least one Semi-Hybrid in the Rolemaster (2) Companions. The Macabre, IIRC. I don't have time to look up which book it was in, but it was one of the numbered RMCs.
RoCo III, p11. Its author didn't go with any skill problem as he gave them the development costs of the Necromancer. Having created one (with alteration of the spell lists and level bonuses), I'd say that its main problem in RM2 was the cost of its... spell lists! Even more if you play with the Extended Chart provided in the SUC! Other than that, it was an okay profession, not worse anyway than many other, and interesting enough in its concept for its existence to hold a meaning. (OTOH, as written in its description, it's foremost a NPC profession, not a PC one)
From my personal experience with the current Semis though, the last two points will hardly have any influence on the development of such a Semi (Hybrid/Arms) Spell User character, because Semis in our games tend to learn almost only their own Base Lists, simply due to lack of DPs. But then again why have Semi-Hybrids anyway, if the one thing that is interesting about being a Hybrid, i.e. learning spell from the Open & Closed lists from two realms, is only in rare cases going to happen anyway
I'd have to agree with this feeling, if one follows the RAW, though an Hybrid can
also channel and be channeled spells and PPs from his two Realms, which may have an interest (though probably limited since a semi-user don't usually have enough DPs to spend on Channeling, having to focus on both magical and combat skills, since, well, it's not a very often use skill).
(In my world and game system, though, it's the most important point since I don't class spell lists in Realms other than Arcane -- i.e. you only have base lists, open lists, closed lists and Arcane lists. Your Realm "only" determine the way you get your PP... which is important enough for someone to wish to be an Hybrid)