Consider three characters; an Arms realm character, a Semi and a Pure caster.
They are foolishly (and rather contrivedly) standing in a field, being attacked by an also suspiciously contrived group of 3 goblins. The goblins each fire short bows on our intrepid heroes, who return with their own favourite ranged attacks.
The first hero, Mr No-Spells-for-Me, has good armour and is well trained with his bow, and fires without penalty. He doesn't have great defence from the ranged bow, but has a few ranks in Movement so gets some defence. Life could be better, but his great OB in his primary or secondary weapon class means he is in a much better situation than goblin 1 will be shortly.
The second hero, Mrs Bows-And-Spells-FTW, has okay armour, pretty good with their bow and has a couple of spells for defence. They rattle off a Shield spell on the first round, and makes their attack as usual. Second round they use a deflect I, or a Turn Missile. Otherwise they use a spell to boost their OB. All-in-all, as long as their PP holds out, it's the goblins are going to have a bad day. Importantly their spell casting is a great benefit to their primary attack.
Our third hero, Mr Blast-it-to-Ash, is also under fire. They have no useful armour, but they have a number of awesome directed spells. Of course goblin 3 is also trying to introduce them to a number of "Single Arrows in their Area". They have a similar offensive capabilities to the others. However if they use a defensive spell... they can't cast a spell (optional rule allows them to cast at -50 on Spell Casting roll AND attack roll).
So the result is that that a Semi can make best use of spells in this situation. Instant spells don't interfere with non-spell attacks. However for a pure, they pretty much need to choose between using an attack spell or a defensive spell. There are other examples with Monks using movement spells to move within (or out) of attack range as another example.
Should an instant spell count as a spell for the 1/round limit? Or should the limit be 1 instant and 1 non-instant per round?