Of course, if people weren't using mIRC to download a lot of large, illegal files, this wouldn't be an issue. I mean, if someone downloads a family picture or a friend's picture, if it fails, it isn't going to take long to download it again. Not that you're likely to put pictures like that on XDCC anyhow. Very few files that are large enough to make this an issue are legal to download. And your comment for 100GB of lost data suggests that you're definitely downloading illegal files (most likely either games or movies or apps). As the sticky says, this isn't meant for illegally downloading files and we're not here to help people to do so. Checksums may be helpful on the rare legitimate occasion, but because it's rare, they probably won't be added to mIRC. Doing so just helps to promote piracy.
Besides, if you know what you're doing, it really isn't difficult to completely download something without such problems.
Also, keep in mind that XDCC is a SCRIPT. It is not built into mIRC itself. It just uses the DCC protocol to send files. The only way MD5 built into mIRC would work would be with the RESUME option. And that's DCC related, not XDCC. XDCC scripts have to requirement to list the MD5 even if mIRC has something built in for it.