They would be able to do so... And get access to level 1 grafts only. I did specify sorcerer spell levels in the first paragraph, after all, so a 1 level dip gets you level 1 sorcerer casting, which gives you level 1 grafts. It's not the most impressive of things to the wizard, cleric, druid or bard, and even the warlock isn't all that impacted by it. Grafts basically function like a spell book, you know, so a 19 cleric/ 1 rune mage sorc can't cast wish any more than a 19 cleric/ 1 wizard.
At level 20, it allows you to shuffle one spell per short rest without damaging your sorc point pool
as much. Before then, it costs an action and 4 points
per spell graft changed, and that is a hefty price even in the end game. Even at level 20, you would only be using rune exchange if you were absolutely
certain you needed that particular spell graft at that particular time. I dom't see it as too much of a problem, since while expensive, it is useful enough to justify its existance ("if only I had prepared X" comes up way to often in game, from my experience). Plus it fits the flexability fluff and I happen to like it.