From the perspective of indie music, they are the easiest way to get on iTunes. If you are looking for a simple way to get onto a lot of shops then that is your best bet.
I've dealt with a number of aggregators and have mixed opinions, though this is not the place to start listing who I like and don't like.
On the up side an aggregator allows you to access a vastly wider range of online retail outlets than would be financially viable to deal with all directly without some form of automated system. For this purpose most aggregators are good.
However, where I feel there is an issue with aggregation is that they are effectively the go between you and the shops which gives you as a label limited control over how well you are pushed (if at all). This is not a criticism of the aggregators, many who have several thousand labels on the books, it would be impossible to act in the interests of all of these.
I personally work with both aggregators and many of the main shops directly. I think the direct rapport is great if you can put the time into it and pays much more than the cut the aggregator would have taken.
Rob is right that anggregators contractually gain exclusivity to the people they supply, but most aggregators allow shops to be excluded from there list of deliverance. That said a lot of aggregators are trying to now include certain non-excludable shops in the contracts as they account for too big a part of there income stream.
Anyway I'm drifting slightly OT here. The short answer to the original post is "no" iTunes only deal directly with large scale labels.