I have to politely agree and disagree.
RoundCube has some issues with memory and attachments; however, not all of this is RoundCube's fault. Many PHP setups are "vanilla" and limit post to 2M and memory to 32M. Downloading large attachments can spiral out of control (a RC bug most likely). Uploading attachments have about 3 different limits to check through including the post upload limit, the HTML form max upload limit, and the memory limit. So while RC may have issues with uploads, it's not always RoundCube's fault. There are limits to sizes of sending mail, and while your limits may be high (or non-existant) not everyone has control over those limits (read: shared hosting customers). So while you may not experience them on your servers with other clients, it's not definitively a RoundCube issue because many factors contribute to attachment sending issues. Although, it does look like RoundCube could have an issue of attachment handling in general.
That is all not to mention that 0.1rc2 is a rather old release. There may have been some improvements to attachment handling.
Perhaps there's a good hack in the midst of the future which can get the max_upload_size and max_post_size and the max size of attachments for the SMTP server and show the smallest one in the attachment pane. Something like:
[ Attachment Limits: 0.4/2 MB ]
Then users will know what their limit is per whatever their server is. Question is, what SMTP servers advertise what their limit is when queried (like the IMAP CAPABILITIES response).