Is it advisable to upload theses to the arXiv? I'm currently uploading my MRes thesis to my university website, but I have some doubts regarding the permanence of the link as it may change after I graduate. I do not have many illusions over who may want to read my thesis in the future, but I would like to have a link to give to people or to put on papers in the future without fear of the link breaking in the near/mid/far future. Additionally, I know that having it available in a place spiders are more likely to look makes it (marginally) more visible to web searches and thus more discoverable.

A quick search turns up a good bunch of PhD theses, but it doesn't say how many there aren't very many MSc or BSc theses in there. My reading on this is that it is OK but by no means a standard practice.

Before I upload it though, I would like to know if there are reasons not to put it in such a public place. I am after all a bit of an arXiv fanatic and there may be factors I'm not considering. Any thoughts?
There doesn't seem to be any real restriction on publishing theses of any kind to arXiv. Therefore, whether you choose to do so is a decision that should be made between you and your advisor, taking into account any regulations your school may have. If your work has been funded by an external agency or company, you should also take their requirements under advisement.

However, so long as your topic falls under arXiv's category guidelines, and there are no restrictions preventing you, there isn't much to lose by submitting the final version of your thesis to a public repository. It would ensure far greater visibility for your thesis than if it's left on your university's website alone.