I have a mixed view on that particular topic - art is important in that
- it provides space in the flow of text that allows the reader to breathe somehow, and
- it provides atmosphere (much more efficiently than, say, a piece of fiction)
The first point can be achieved with almost any kind of art (even placeholders). However, getting any kind of art will likely make a mess of the second point and eventually detract from the reading if the art clashes too much with the text. thus, the problem often comes from the second point.
I don't think that colour is necessary - you can have very interesting and atmospheric B&W art, which gives you the option to keep the printing in B/W completely, which I assume is a cost reduction. Off the top of my head, there are a number of B/W art pieces in other RPGs I found very tasteful and atmospheric (for example, David Interdonato's art in Arrowflight, some of William O'Connor's art in Aria Worlds and Ars Magica 3, and even most of the art in Apocalypse world which fits the atmosphere perfectly and was probably derived from stock art). Note that Arrowflight is also a good example of how mixing art can have a negative effect overall - the various sources clash completely and seem to be pulling in different atmospheric directions.
Adding colour to ill-chosen art is not a saving grace. On the other hand, using colour in a coherent way can provide wonderful results, and one of my fondest memories of "colour art defining an atmosphere" is DiTeerlizzi's art for Planescape. This kind of art direction is probably out of reach for ICE, though.
Basically, art direction (what kind of art, the atmosphere it must provide, the overall artistic consistency if several artists are involved - remember CCGs where some cards had gorgeous art while others made you cringe, what each piece must be about to fit with the surrounding text) is the critical point here. I believe limiting the number of artists involved is preferable (and there is obviously a trade-off between reducing the number of artists and respecting deadlines), and, given the time it took for RMU to hit the shelves, ICE was actually in a favourable position here as long as they could procure art early enough.
But, to be honest, I was never particularly impressed with Rolemaster interior art, and it never struck me as something the editor deemed essential. It was obviously partly a question of budget, but I don't think most of the indie games have more budget than ICE does, and some of them are really good anyway. I think it was also part of the mindset: the RM rules come from a minimalistic foundation (d100 OE, open skill bonus, roll high) but since every single situation seemed to require its own custom mini-mechanism or tweak, the rule text becomes bloated and an incredible amount of effort goes into trying to stabilise that text and make all pieces more or less fit together. And since a limited budget implies a limited amount of effort, art direction was taking a back seat.
I cannot comment on RMU art directly (I don't plan on buying it, since the game rules and my tastes are moving in very different directions), but DTRPG provides a low-res version of the cover art and, well, I can't say I'm impressed. Aiming for a more stripped down cover art (L5R 3rd, 4th, or even beta 5th edition core books, for instance) would have worked better. But that's just me.
