Utilities are worth buying, especially early in the game (before the house building phase starts. Their main drawback is that unlike the color groups, they can't be upgraded further.
Consider Electric Co., which sells for $150, and is sitting between St. Charles Place (cost:$140) and Virginia Ave. (cost: $160). But the average rent on Electric is $28 (4 x 7, the average die roll), versus $10 for St. Charles and $12 for Virginia. Only if the purchase of one or the other maroon completes the monopoly is Electric Co. worth less.
Likewise, the two utilities together cost $300, but an average rent of $70 (10x 7, the average die roll). That's more than the $50 you can collect on Boardwalk, which costs $400.
Two utilities are even worth more than two railroads ($50 rent at a cost of $400). It's the third, and particularly the fourth railroad that really adds value to them.