5th edition made blast weapons a lot better. Their chances of a direct hit went way up with the ballistic skill reduction in scatter (for example, a BS4 blast went from a 33% chance to hit to a 50% chance to hit and a 2.7% chance to scatter 2" or less to 25% chance to scatter 2" or less) though barrage weapons without line of sight do not gain this benefit. Furthermore, all models under the blast are automatically hit compared to previous editions where models not fully under the blast were only hit on a 4+. However, outside of a few units in particular armies (ahem, Imperial Guard) we don't really see many units based upon blasts seeing a lot of playing time. Is this because the units themselves aren't very good or are blast weapons being overlooked? At the same time though, you can see a few select armies based on blasts (i.e. Imperial Guard Leafblower) which are perhaps taking the concept too far. Let's take a look and break this down further.
Blast Advantages -
Clumped opponents hate blasts. Even parking lots of tanks don't like to see lots of blasts if they can penetrate the tank. Whilst half strength of most blasts isn't going to harm AV11, having a bunch of tanks parked closely together means blasts are going to have a much harder time missing. Chuck in some recently disembarked infantry or infantry hiding behind tanks and blasts are going to do massive amounts of damage. This is also important in that it allows the owner of the blast weapon to target infantry normally out of Line of Sight when hiding behind vehicles. Drop the blast on the edge of the tank so it clips the infantry and if you roll a hit or a small deviation - infantry normally not seen take damage.
Ultimately, blasts are about damaging infantry though, particularly clumped infantry. Infantry who have recently deep struck, won combat and not consolidated far, disembarked or basically been forced into a tight formation (terrain, tank shocks, lash, etc.) are prime fodder for blasts as they can generate a large number of hits.
Blast Disadvantages -
The problem is, the above situations are rare against a smarter opponent. If you have strong blasts in your army (i.e. a S10 ordnance blast), your opponent is unlikely to clump up but spread their units out - particularly parking lots. Even leaving 1/2" - 1" gaps (which most parking lots should have to ensure vehicles can pivot) reduces the accuracy of blasts by a decent amount. Deployed infantry are going to spread out (you'd be surprised how much a couple inches from running or consolidating can spread infantry out for example) and you're only really going to get tons of infantry under the template after specific situations such as disembarking (forced or otherwise).
You're also relying on a scatter roll. You can place the template perfectly and even with a tiny deviation in the wrong direction, miss completely or get less models than your original position.
The Verdict -
Blasts have a very high damage potential and if they can hit clumped infantry or tanks (particularly ordnance blasts), their damage ceiling is huge. Smart opponents are going to avoid these situations however and simply spread out. This is where blasts really come into play - they force your opponent to spread out. They will certainly do damage to your opponent and if you can catch them with their pants around their ankles such as during deployment or after disembarking, etc., they'll likely do a lot of damage. Beyond this though, it forces your opponent to use more of the board. This spreads their army out and allows you more tactical options and if they don't, well they could suffer high strength blasts on lots of their stuff.
Obviously there are some blasts which do this better than other. A Manticore is going to wreck a parking lot with D3 S10 Ordnance barrage blasts whilst a missile launcher frag missile is only going to scare disembarked infantry. Running a bunch of missile launchers isn't going to make an opponent spread their tanks out - a Manticore will.
The end result is without impressive blasts to force your opponent to spread out, you lose a bit of utility and ability to impact your opponent's movement (and how are games won again?). This isn't that huge - a list without blasts isn't bad for example, but there will be times when an army without blasts really wishes they had an impressive blast weapon. Should you try to design armies with such weapons in mind then? Certainly, as long as it doesn't come at the expense of something else. Vindicators for example have a very scary S10 ordnance blast weapon but the platform they come on and the points they pay often isn't worth it (Blood Angels can be a different story). In the end, if you can fit a couple of significant blast weapons into your lists, all the better but don't go out of your way to force them in. They provide nice utility but like all utility they won't win you games alone.
You can however go too far in the blast department. Blast effectiveness goes down as there are less models on the table. Therefore, taking a couple of blast weapons will see they perform at their peak efficiency when their moment comes(outside of the utility they provide). If however you are taking lots of blast weapons, they begin to give decreasing returns as each gun fired has a lower damage potential ceiling. Lists which therefore base significant portions of their damage on blasts can do a bucket load of damage but can also be over paying for their services. In this regard, purchasing a couple of units with significant blast weapons in mind will generate more bang for your buck than going overboard.
