It's true that the cost of sequencing continues to follow Moore's law -- exponential decline in material and equipment costs and increased computing power -- so that what Illumina does with supercomputers is something that doctors and patients will be able to do with smartphones. But Roche is not interesting in the hardware of sequencing. If it did, it could just buy a bunch of machines for a lot less that $5.7 billion.
In fact, Roche knows that sequencing will become commonplace. I think it wants Illumina because of it's partnerships with clinical labs worldwide to perform sequencing. Distribution for Roche diagnostics and products -- as the firm focuses on targeted therapy -- is more valuable than equipment or sequencing. Roche also sells glucose kits too and find that marketplace pretty lucractive.
Should Illumina sell? I don't know and have no opinion. However, I remember when Yahoo snubbed a Microsoft acquisition for a hefty premium over it's share price. Now Yahoo is scrambling to find a buy at a share price far below that offer. Information is quickly becoming a commodity in health care. In 10 years drug companies will be delivering individual content directly to consumers to treat illness. Perhaps Roche believes it can be a source of revenue growth. In any event, acquiring Illumina is about how medical care will be organized in the future, not about it's microarrays now.