I have seen some contractors bash the use of sealers on their websites. They try to sell you the idea that the contractors that are using sealers are just trying to skimp on costs by skipping the first coat of polyurethane. These contractors are simply misinformed. Manufacturers and the National Wood Flooring Association alike both strongly recommend the use of sealers, especially when using water-based polyurethanes.
For those who don't know, a sealer is a coat that is applied before the final step of polyurethane; it can be applied to raw wood, for a natural look, or over the stain of your choice. It is recommended to use one to two coats of sealer followed by two coats of polyurethane, as opposed to the alternative three-coat of poly method. Water-based polyurethane is actually strong enough that, when it is applied and seep into the cracks, it act as a glue and bond the boards together; this is called side bonding and, when done in large chunks, it is called panelization (see photo below). You run into a problem when the wood shrinks, in the low humidity months, and pulls apart from itself in large chunks. Using a sealer is one way to help prevent this problem. Another way to avoid that type of shrinkage is to run a whole home humidifier and to keep your house between 35-55% relative humidity.
Tannin bleed is an issue that occurs when water-based polyurethanes pull up the tannins in the wood and leave stains. Sealers are a great help in preventing this problem, which can frequently occur in wood species with a large amount of naturally occurring tannins like white oak.
If you fail to follow the manufacturers process, you run the risk of suffering serious damage to your new floors, including shifting and splitting of boards and ugly stains. My advice is to do your research. Manufacturers stress the importance of sealers for a reason, and the benefits of following this protocol far outweigh the risks you run by neglecting this step in the process.