D&D: 1st ed 12 years, 2nd edition 10-11 years, 3rd edition 8 years, 4th edition 2 and counting...
Warhammer: 1st ed 16 years, 2nd edition 8 years, 3rd 1+
GURPS: A couple of editions up front then 3rd ran 16 years, 4th is at 6 years and counting
Hero: 3 editions that were all very compatible over 8 years then 4th ran for 12 years, 5th then ran for 7 years and we're on 6th as of the end of 2009.
Shadowrun: 1st ed - 3 years, 2nd edition 6 years, 3rd edition 7 years, 4th is at 5 years and counting
Rifts: Pretty much 1 edition for 20 years with some small tweaks in the Ultimate Edition a few years ago.
M&M 1st edition 3 years, 2nd edition 5 years with a 3rd ed on the way.
So yeah, 3 editions in 5 years seems like a lot, and the changes from 1st to 2nd were mechanically significant to the point that 1st ed stats were not all that useful for 2nd edition as anything other than a guideline for conversion.
A new print run has been a cause for a new edition with some games in the past. This is what happened with Hero in the early 80's/. When stocks run low and a new print run is ordered on a fairly small game (anything other than D&D) the publisher might as well make a few rules updates to get them out into circulation. Nothing wrong with that and it meant that Hero products were directly compatible for close to 20 years.
I think some of the reasons for this might also be tied to Green Ronin being a smaller company. History has shown that the core book has the greatest sales in a game line will come from the main rule book. We also have seen that the best way to spur sales of a rule book is to make a new edition. So I think it's fair to speculate that a revision of their #1 game line will help the financial situation.
They have also stated that with the DC license made it look like a good time for a new edition, making both lines completely cross-compatible. That makes sense too and it's probably better for the long-term health of the game assuming there is some influx of new players attracted by the DC universe license.
The downside of these edition changes is that current players have to decide when/how//if to change over. There's a financial cost, there's the time cost of learning a new system, there's usually some loss of "system mastery" to those involved, and a conversion cost of making use (or not) of the other books you already have with the new system.
In general, I'm OK with edition changes when they address some specific issue or issues with a system or if a system has had a good long run and everyone is ready for a change. I realize that the first is pretty objective while the second is not. An example of system issues is the change from Shadowrun 1st to Shadowrun 2nd - there were some major mechanical issues and some rethinking of how to handle things once the game was released and the 2nd edition was a much better system mechanically and had a nice run. System fatigue/sprawl is best exemplified by D&D 2nd edition. By 1999 there were a ton of core books, kit books, adventures, campaign sets and odd sort-of-supplements (Volo's guides etc) and it was time for a reboot. After 10 years who can complain?
I am really not on board when a new edition just feels premature - either the game rules are incomplete, the system seems to work just fine, or the time since the last edition just feels too short. Warhammer Faantasy Roleplay moving from 2nd edition to 3rd edition feels like this to me - 2nd was a great system, well supported, but there was plenty of material left to explore - Where was the elf book? Where was the Lustria book? How about the undead lands to the south? The Warhammer World is rich place dripping with detail and I did not see a need to switch systems (and publishers) that quickly. Star Wars d20 has been a bit of a mess here too with 3 editions in about 8 years, basically 1 for each movie of the new trilogy as they came out. 1st ed only had about 5-6 supplements but 2nd, the RCR, was a full-on game line with around 10 hardcover supporting books that had a lot of good material. The 3rd, Saga is by far the best system and has the most support but ends up with a lifespan of only 3 years!
This beings up one more side effect of a new edition - the "supplement tax". When a new edition of a game comes out, you can bet that there will also be new editions of the supplements you already own,with the worst cases giving you the wonderful experience of paying for something you already bought once. I have 4 rigger books for Shadowrun - 1st, 2nd, 3rd, and 3rd revised which was practically a new edition, and that was over about 10 years! I have 2 versions of the Freedom City sourcebook for M&M that only came out about 5 years apart. I bought a multitude of class books for 2nd edition D&D , D&D 3rd edition came out with 5 class books in the early 2000's, then for 3.5 they redid them as 4 hardback books, and now I've been picking up power books (the 4e equivalent) for 4th edition. Its' one of the most annoying parts of an edition change. That said it's also where a game can really shine so it's not something that's easily ignored.
So to M&M specifically: To me it feels like it's too soon. Doing a revamp only 3 years after 1st edition should buy more time than 5 years. I bought quite a bit of 1st edition M&M's supplements and felt a little backstabbed when they announced 2nd edition so quickly. The changes in 2E were good though so I got on board and I've been slowly picking up the books as my 3rd or 4th priority behind D&D 3.5/4E and Star Wars Saga and Traveller or Savage Worlds. Now that I have a fair collection of 2E books, we're going to 3rd. I expect we'll see another Freedom City, another Lockdown, another bad guy book, another good guy book, some more "age" books. It's not bad I'm just not sure it's necessary. How is it going to make my game better? That's what I need to hear. I'll be haunting the forums to watch this one develop.