Of course, you don't have to have an agent to have a deal with a mainstream publisher. Currently I am both 'Indie' (self published via Kindle) and 'mainstream' (I have a programming paperback book due out in July with No Starch Press). So far I can see advantages and disadvantages in both routes:
1) Indie I have the last say in everything/ mainstream I have to arrive at compromises (for example on style) with my publisher
2) Indie I have to do all the proofreading and layout myself/ mainstream I have about 5 or 6 different people working on proofing and design
3) Indie I have to do all the marketing/ mainstream they have a marketing department, a network of distributors etc.
My ideal compromise would be to have all the creative control that Indie gives me with access to the proofing, design and marketing of a commercial publisher. I suspect that very few authors ever achieve that! Â

As for my Kindle novel, would I respond favourably to an approach from an agent or publisher? If it gets me more readers, absolutely.