I think your perception of value is going to be heavily influenced by your level of SEO expertise and what your SEO workflow looks like. Aaron Wall has a great pyramid for expertise in an arbitrary market -- the beginners always outnumber the intermediates who outnumber... SEOMoz Pro is targeted at, if I don't miss my guess, high beginning/low intermediate. If you can't look at the SERP for [bingo cards] and say "This is competitive but not nearly as competitive as [student credit cards]" then they have a tool which will tell you exactly that and that provides value. For businesses who are in that sweet spot it is totally a no-brainer.
With regards to the workflow bit: SEO is primarily a game of link acquisition at the competitive levels. One common strategy for acquiring them is to identify pages on the Internet that would be good links and then email the people in charge of them to ask for the link. A lot of the SEOMoz tools are designed to make this process more efficient. I have deep misgivings about it (from both a it-is-a-nuisance and the-ROI-is-inferior-to-less-annoying-alternatives perspectives) and don't practice it myself, so those aren't so useful for me.
I would like to flatter myself and say I'm a wee bit above low-intermediate in SEO. I mostly use the Rank Tracker, and the other tools have not frequently given me actionable information. That said, I am not displeased with my $80 a month or whatever it was. If there is ONE thing I wish everyone at HN understood in their bones about SEO, it is that the ROI on it is just freaking amazing. Pick a number beyond your wildest dreams, no, it is better than that. In that context, I often spend speculatively on it, and ~100 a month is easy to justify even if it only results in positive decisions once a year.
(Edited to add: Don't let the Internet Marketing crowd fleece you, though.)