Using short url won't be considered as a spam. But it won't help you anyways in seo. Moreover, it can restrict some incoming benefits. For example, i use to keep short urls of my site address to bookmark, but on searching the bookmark site with the actual url, i found that it was already bookmarked by 10 other users.
Short URLs have their purpose -- they can be easy-to-remember and easy to link to on Twitter or from a mobile device. But they use 301 redirects to redirect the user to the correct full URL, and while 301 redirects DO pass pagerank, they don't pass complete pagerank (a little is lost each time a 301 redirect is used). If you don't explicitly need to (b/c you're on twitter, etc), don't use them.