There are no law society guidelines on whether or not solicitors should act as executor.
A solicitor is often a good choice to act as executor. They are aware of the liability an executor is exposed to, they are not a beneficiary, they are neutral to any family feuds that may be going on, they know all there is to know about probate/wills/CAT and tax planning and any land transmissions necessary. It is in the best interest of the solicitor to finalise the estate quickly, in order to obtain their fee and also keep the beneficiaries ( potential clients) happy and show them a good service. Nobody will feel passed up if the solicitor is the executor ( sometimes one sibling might feel hard done by if another is the chosen one).
When I take instructions for a will I ask the testator who they would like to appoint as executor. If they don't know or understand fully what an executor does then I explain it- this is quite common. Sometimes people come in with a pre-concieved notion that their solicitor must be the executor, this is quickly disabused. Some people come in and clearly have a difficulty choosing a family member to act. I would normally present them with different options- a family member, spouse, trusted friend, accountant, solicitor. It is entirely up to the testator and everything is explained fully to them.
The suggestion by Brendan is that a solicitor shouldnt act as there is a 'conflict of interest'. This is, I assume, because of the fact that if a solicitor is sole executor then they can insist that their firm deals with the probate and therefore can charge whatever fee they like. This is simplistic and, to me, another example of Brendan's antagonistic attitude to solcitors lately.
It is true that if a solicitor is appointed sole executor that they CAN insist their firm deals with the probate. However in reality, if there is family opposition it is more than likely that most solicitors would simply renounce. Even if they didn't and there was a contention by the family that the fee charged was too high, the beneficiaries have recourse to either the law society or the taxing master.
Although it may be hard to believe in this anti-solicitor age, there are many people for whom their solicitor is someone they trust to carry out their requests after they die, and to whom, allowing them to appoint their solicitor their executor gives them great peace of mind.