I generally buy DLC for the same reason I buy games and other forms of entertainment - because I expect I will get my money's worth (or more). I do understand why some would have a motivation (fully or partially) based on collecting, but it's not one I have myself.
There is a however another reason I sometimes buy DLC, one that is not listed in the poll: To support the developer. As a gamer and a fellow developer I think DLC is a great way to be able to financially support game developers. In these cases I will buy DLC (or even full games/expansions) that I would not otherwise buy, if I did not have a desire to support the developer. A variation of this is when players buy a DLC to increase the likelihood of future support/sequels/similar games from a developer/publisher. There is also a negative variation of this, where players actually do think the content is worth the price, but they want to punish the developer for something directly or indirectly related to the DLC.
As others have mentioned I think the "it's cheap, why not?" option is identical to the first option of value for money. Personal finances dictate how much value we require for our money. For some people, a very low price can mean that the very act of purchasing is worth the price (the value of performing the purchase can be surprisingly high, as shown by how common impulse buying is), or at least adds enough value to the actual product, that purchase becomes worthwhile. Incidentally, the pleasure of an impulse purchase is rather short-lived, and for some people prone to causing buyer's remorse - no different than other luxuries really.
I also think "I wanted the bonuses" is very much the same thing as "I thought the DLC was worthwhile and I bought it", unless we're talking about a competitive game. The poll option doesn't mention competitive/multi-player gaming, so I have to assume it's not the intent.
For me, a better breakdown of motivations for buying (or refusing to buy) DLC would be:
1) Content is worth the price.
2) Desire to add the DLC to one's collection.
3) DLC gives a power advantage in a competitive game worth the price.
4) Desire to show off cosmetic DLC in a multi-player setting.
5) Desire to show off DLC to visiting friends (probably not very common)
5) Desire to financially support the developer.
6) (no buy) Desire to financially punish the developer.
7) (no buy) Ideological opposition to this DLC (in particular or in general).
Also, it is not a binary state as to whether a purchase is worthwhile. I may think something is really good value for money or we may think something is expensive, but still within the bounds of what we can accept. As with other purchases, players can also be disappointed at the DLC not delivering as much value as they expected. This happens fairly often for me, and a developer having a history of this, is of course something that can dissuade me from making a purchase.
Another relevant factor is what expectations players have in terms of content updates assumed to be part of purchasing the base game. If the developer promises a lot of content patches for no additional price, and does not deliver on this, players will generally be really unhappy about any paid DLC. Conversely, delivering quality free content updates tends to make players more accepting of paid DLC.