The news confirmed yesterday (19 January) that Pinsent Masons and McGrigors are in advanced merger talks suggests 2012 is set to be a bumper year for law firm M&A. With discussions between DWF and Cobbetts emerging in November and Legal Week this week establishing the existence of the much-rumoured talks between Herbert Smith and Australian leader Freehills, the urge to merge is obviously spread across widely varying sections of the legal industry. And that's before you get into the frenzied round of merger gossip currently doing the rounds.

If there is one thing uniting these proposed tie-ups, it's that they all leave you somewhat scratching your head as to the logic (which doesn't necessarily mean the logic isn't there. Some initially lauded unions flop while other little-loved combinations deliver in spades. Back in 2001 when Berwin Leighton and Paisner & Co tied the knot, they were both viewed as so sleepy that rivals could barely be bothered to raise a patronising eyebrow).

Of the three, the most imaginative union is that between Herbert Smith and Freehills. As with Ashurst's Blake Dawson tie-up, you can see the logic in terms of building out a global platform and bulking up in the Asia-Pacific region. But just because a tie-up between a top-tier Australian practice and a strong City player works in theory doesn't mean it will gel with this particular pair.

Has Herbert Smith got the stomach for the upheaval, sacrifices and grind of such a large union? It's always been light on central management and has precious little experience of such deals, while the firm is at something of an existential cross-roads. While you could argue that it would help Herbert Smith pull off the cultural reboot that it needs, ultimately it first has to be ready to take that step on its own – a huge foreign merger can not make the mental jump for it.

Meanwhile, for connoisseurs of legal M&A, Pinsents and McGrigors both have plenty of form. Pinsents is the product of three sizeable UK mergers – one regional, one London, one national – which yielded mixed results. The original marriage between Birmingham's Pinsent & Co and Leeds' Simpson Curtis was so bloody that some felt it cost Pinsents its position as the best national prospect to take on City law firms' transactional monopoly in the 1990s.

Subsequent deals with Biddle (2001) and Masons (2004) were considerably more successful and the firm has been a consistent performer over recent years, even if some believe that the commanding position in technology and construction it had amassed after the Masons union could have been more effectively built upon.

One notable challenge for Pinsents is both irritatingly superficial and annoying meaningful. Two of its obvious peers – DLA Piper and Eversheds – are not only much larger but among the most polished, shiny and high-profile legal brands in Europe. Judged in financial terms over the last five years against DLA and Eversheds, you can't really fault Pinsents (aside from the drift-inducing and now-abandoned alliance with Salans). But the Pinsents name needs to carry a little more resonance in the upper mid-market range it wants to hunt in. The firm would certainly benefit from raising its profile among bluechip clients, and scale and international reach would clearly help in this regard.

McGrigors has some similar issues. Like all the big Scots firms it has suffered from the slow erosion of Scotland as a professional service hub and the challenge of competing next to the far larger English market. Both McGrigors and old rival Dundas & Wilson have also had to contend with bad luck in that their once-promising tie-ups with accountancy giants were torpedoed by the indirect fallout from the Enron scandal. Having lost valuable ground, McGrigors has been on the hunt for a City merger for years.

Under managing partner Richard Masters, the Scots firm has been proactive, securing a merger with City boutique Reid Minty in 2008 and expanding in Manchester and Belfast. But while the firm has a strong reputation in projects, energy and infrastructure and has been a decent performer financially – putting ground between itself and Dundas – it's still hard to see a future that bright for a full service law firm in its weight class.

So what's in it for McGrigors is fairly clear: a deal with Pinsents provides scale, investing power and becoming part of a firm able to build an international platform. There is also a respectable practice cross-over.

The pitch for Pinsents is less immediately apparent. For obvious reasons, its focus is on London and international expansion – not national growth. And McGrigors' £70m turnover doesn't look a sweet enough incentive to make the hassle of integrating two larges UK practices worthwhile, given that Pinsents already has substantial resources to fund its growth.

But it may come down to the basic calculation that getting bigger through merger is better than strategic trudge and continuing to slog on alone in what remains a very fragmented market. If you follow that logic – and there is something to it – then ultimately issues like culture, practice fit and even business case just don't matter as much as they did.

So far they have mattered. Over the last 15 years, endless rounds of merger talks have only resulted in a tiny number of deals within the top 50 (off the top of my head, the roll call is Addleshaws/Theodore Goddard; Wilde Sapte/Denton Hall; Pinsents/Masons; Hammonds/Edge Ellison and Clydes/BLG).

You can forget all the forensic stuff about strategic tie-ups; large UK mergers usually don't make it over the line because the partners haven't wanted them enough and didn't sufficiently fear the consequences of not doing a deal. It's the subtle difference between the question on the lips of managing partners being not "why merge?" and rather "why not merge?" If that's the mindset we are getting to now – and I'll believe it when I see it – the legal market is set for one hell of a shake-out.

• Related: Pinsents and McGrigors in merger talks over near-£300m tie-up