To say that a summary is important to a work of fiction is an understatement. The summary is almost certainly the first thing your prospective reader will see; it is where they go to know what the story's about and to get some idea of whether they will want to read it or not. You are essentially making a pitch for their time (and money, if you're writing professionally), so your summary is a vital way of letting them know that spending it on your work won't be a waste. So making the first thing that your reader sees a statement declaring that you aren't any good at an essential part of writing is a bit like introducing yourself to a prospective partner at a blind-date by saying that you're terrible at introducing yourself and then proceeding to list all your worst character flaws.
Most online works described in this fashion aren't very good. Furthermore, such stories usually die long before they are completed, due to the author giving up from lack of feedback. Neither is surprising; if the author lacks the skill to write a proper summary, the story itself usually doesn't fare any better.
Sometimes followed with "please read and review", although most fanfic readers take this phrase as an ominous warning and end up not even reading the fanfic, let alone giving any feedback. Sometimes, the phrase is substituted with "Summary sucks, story is much better", but most readers will react in the same way as before.
"I Suck at Summaries" should be avoided since it tells readers that either the author doesn't have any self-confidence, or that the story is so boring that the author couldn't be bothered to put in a real summary. Or worse, that the author doesn't suck at summaries so much as they suck at writing in general. After all, if the author can't even write a basic two-sentence summary, then why should the reader have any confidence in their ability to create a longer narrative? It also shows a failure of Show, Don't Tell: don't use the summary to say the summary is rubbish - write a better one that actually summarizes the entire story.
Some writers attempt to justify this when on websites like fanfiction.net where the summary limit is under 500 characters by saying they don't have enough space. This isn't much of an argument, since the whole purpose of a summary is to establish what the story is about in a concise manner without revealing too much of the plot; long summaries defeat the whole point. Giving an eloquent summary of your story in a short space is actually a show of good writing. Besides, 500 characters isn't that short.note
All this being said, one should always remember that Tropes Are Tools. Sometimes condensing a full-length story into a few sentences can be quite difficult, especially where more complex plots are involved. Long-Runners have a similar issue; when a fic is a couple hundred thousand words or more in length, something is bound to be lost in the compression, and it can be hard enough to decide what to mention and what to leave out.note It's also worth noting that writing a story and condensing it into an effective summary are not the same thing, and it is quite possible to be good at one and bad at the other. There are no universal indicators of quality or a lack thereof, and while sucking at summaries is usually a warning sign, it isn't necessarily always so.