In more abstract games like Heroquest I could see not using it, but in a game like RQ where some attempt is made to simulate reality, it's important to me to have this option.
Ah, my friend, such was the world back when that RQ -- and indeed D&D and most all other RPGs -- never used a grid-based system. Heck, most didn't even use miniatures as part of the game system. Which is especially interesting given that D&D, the granddaddy of them all, evolved out of a miniatures-based wargame.
Don't get too hung up on minis in your game. If you want to use them, that's fine, but I've found that unless the game is explicitly designed around their use in a tactical sense (such as d20 is), then more often then not, they impede the ability to simulate the combat accurately by constraining it too much.
Hmmmm an interesting (but somewhat inaccurate) point of view.
Miniatures have many uses, from 'who was standing where' to 'did that Duck have LOS with the sling' (I know a lot of GM's don't like this - its a power thing I presume).
How is having a representation of something 'constraining' if anything its the opposite, it frees you up to control the situation more effectively.
It reduces player cheating ('no I wasn't standing there, honest') and the GM's 'fluffing' things (for whatever reason).
If you want more realistic combat, then do LARP (where the miniatures are life size Lol) as a tabletop simulation can never be realistic OR accurate (In reality its just as easy to kill a man with a Dagger as it is a Sword - yet swords deal out more damage).
The first RQ product was the wargame White Bear & Red Moon, A wargame with Playing Pieces. A couple of the smaller Miniature manufacturers even produced 'Runequesty' style miniatures, sometime BEFORE the RPG was ever produced (the long gone Asgard Miniatures being the main one).
Your observation that RPG's aren't designed to use miniatures is a spurious one at best - thats a MODERN concept that comes from most Storytelling games (such as those published by White Wolf) and not from RPG's which tend to be more Adventure Based.
The first Editions of Runequest and Tunnels and Trolls have sections in them about "Using Miniatures" in your Games (and Tunnels and Trolls had its own 'Quasi' Miniatures Game called "Monsters, Monsters").
In fact the Strike Rank (and the AP systes from 3rd Ed RQ) are Wargaming conventions, and have been for many years.
Although I agree about the Grid (even though AGAIN there are RPG's that use Grids, Star Frontiers and Dragon Quest being prime examples) they aren't common.
But the use of miniatures IS a big BIG part of Fantasy RPG's (and Science Fiction ones to be honest).
With regards to playing games purely with your miniatures, the RQ combat system as it stands will be more than adequate. You will need some form of points based system to make your games fair however.