Great discussion.
There is nothing stopping a service provider from providing more than 2-3 keywords per package. However, most of the time, a certain group of buyers who are buying these services have a couple of keywords in mind that they are looking to get results for. Blame it on what they have read, blame it on the perception they have built over the years, blame it on anything ... but that's what it is.
So, while a service provider can technically provide more than a couple of keywords per package, the buyer needs to set his expectations right too. He can't expect extreme movements in the course of a week, specially when the links aren't particularly targeted for any set of anchors. He will see results, of course, but that would be long term so while some buyers would want this, others, who are looking for fast results (and there are a whole lot of them) would simply stay away.
There are people on BHW that I've ordered from (mostly those who provide their services elsewhere) and they have done their job. Its me who should know how to use them and what to expect. I'm not blaming it on the clients; being a service provider myself, there are many people who I have worked with who know exactly what they want and we do that for them. However, an equal number of people want results for cheap and want them fast, for rather easy keywords. That, I believe, is lucrative for many service providers, especially those who have just started off.
Of course, you can just price your service higher and filter the tire kickers just based on that. Here, you have to make the decision and most service providers, unfortunately, fall into the trap of doing volume at a lower price. I've seen both ends of the spectrum (selling for dirt cheap and selling for relatively higher prices); we allowed our clients to add as many keywords as they wanted, and encouraged them to diversify their anchor text and sources quite a while ago. But then again, these are people who are already on top of the game and they know what they want.
Many service providers, I believe, run their tests and have their own sites (at least those on the higher end). I have a bunch of my own sites, those that make money as well as those that are used as test platforms. We run our own tests all the time, invest some resources and a lot of time into this just to make sure that we are on top of almost every change. As RevSEO accurately points out, right now, more than ever before, you have to figure in analytics into your SEO. That takes time and effort and subsequently, the costs go up. I do believe that many service providers here know that a better long-term strategy would be anchor variation (taking an example here).
The gist is this: there are different types of buyers. Some know what they are doing. Some don't. Those who do know that it'll take time to see results and know what a good long-term strategy looks like. The service provider, however, is caught between trying to cater to all sorts of buyers and that is where the problem, I believe, lies.
So, in my humble opinion, the ideal solution if the service provider is allowing only a couple of keywords would be this: if you, as a client, want more keywords, ask the service provider if he can accommodate them. More often than not, they will be able to (unless they are stubborn, or are new) and you get your way. It'd take less than a minute.
Here's some food for thought: After allowing a greater number of keywords per order, about 70% of our clients still put in two or less than two keywords when they place an order. We do send them an e-mail letting them know that we will be adding some generic anchors yet, that's an interesting number. One person was furious that we added a couple of generic anchors (this was about a week ago) even when we had e-mailed him (and have this on our offer, through our buying process, on our order form, etc).
Oh, and in case someone forms the wrong perception, I'm totally in favor of allowing more keywords. Its a decision the service provider has to make though.