I think using the Image Sizer plugin is really a “no brainer” as once its set-up in the weblog entries tag, you only ever have to upload an image using the upload manager once, without having to try and remember what dimensions you need to resize the image to every time you upload a new one.
Its also great if ever you need to have differing sizes of the same image across your site. For example with a thumbnail version and also a “full size” version. You would just set up two different Image Sizer tags but only ever have to upload the one image.
Further to this, if your not the one who will be uploading the files and it gets left to a few different people then it makes life so much simpler in basically every way.
Image Sizer is one of those plugins I install with every expression engine install I do as chances are i’m going to need some dynamic image manipulation at some point.
Mark has linked to it, so I would get it, install it and have a play and if after that you choose not to use then fair call.